A short biblical answer to your question is supplied by Peter in Acts. Having heard a sermon that he preached on Pentecost, many were struck to the heart and cried out what shall we do? Peter replied, repent and be baptized every one of you, in the name of Jesus Christ for the forgiveness of your sins. And you will receive the gift of the Holy Spirit. (Acts 2:38) 
 
But this is not simply to be understood as a ritualistic observance that we fulfill on one day, but is meant to usher in a whole renewal of the human person. And thus, we ought to look at all three things that Peter indicates in some more detail. 
 
The word translated as “repent” is Metanoia which means more than to clean up our act. It means to come to a whole new mind, rooted in what God teaches and reveals, with new priorities and able to make better decisions. 
 
To be baptized is not only to be cleansed of our sins but also to see our old self put to death, and for Christ to come alive in us. Baptism ushers in the beginnings of a lifelong healing process that must continue by God's grace. Baptism also points to all the sacraments of the Church. For, having been brought to new life, we must also be fed by the Eucharist and by God's word, we must see the wounds of sins healed in confession, we must be strengthened for a mission by confirmation. Baptism also makes us a member of the body of Christ. And thus, we are called to walk in fellowship with all the members of Christ’s one body, The Church. 
 
St. Peter also speaks of receiving the gift of the Holy Spirit. And thus, we are taught that our dignity is to be swept up into the life, love, and wisdom of God. We are called to be sanctified by the Holy Spirit to see sins put to death and many virtues come alive. 
 
Finally, something needs to be said about your use of the word "exactly" which might imply there is some very simple formula for getting saved. But as can be seen, there are many dimensions to the work of God in saving us. Thus, we are to walk in a loving covenant relationship with the Lord and His body the Church. And like any relationship, this cannot simply be reduced to a few things. We must trust the Lord and walk in a relationship of love and obedience to him. We are to do this in fellowship with His Church, through the grace of the Sacraments, obedience to the Word of God, and prayer (cf Acts 2:42).

Palm Sunday of the Lord’s Passion marks the beginning of Holy Week, the final week of preparation before the feast of Easter. In the Roman Rite, the celebration of Mass has particular traditions that make it look much different than a typical Sunday Mass. Many of these traditions are centuries old, having roots in the early Church, based on the events that occur in the Gospel passages. The differences are meant to enrich our celebration of Jesus’ Passion, immersing us into the events in a unique way to help our souls ponder the beauty and riches of the Paschal mystery. 
 
Why does Mass begin with a procession? 
Besides imitating Jesus’ procession into Jerusalem, the sanctuary of the church is naturally a place that symbolize heaven, with the presence of Jesus in the Eucharist. Furthermore, often the sanctuary is elevated by a number of steps. This too has symbolism, lifting up our eyes (and hearts) to God, but also reminding us of Jesus’ ascent to Mount Calvary. The priest assumes this role and ascends to a "mystical" Mount Calvary to offer the sacrifice of the Mass, participating in the one sacrifice of Jesus on the cross. 
 
Why are palms or other plants used for the procession? 
Biblical scholars often translate the branches people used for Jesus’ triumphal entry in generic terms, such as in the Gospel of Matthew, "The very large crowd spread their cloaks on the road, while others cut branches from the trees and strewed them on the road" (Matthew 21:8). In essence, when palm branches are not available, it is perfectly acceptable to find any type of suitable branch to help commemorate Palm Sunday. The branches are meant to be a symbolic gesture, symbolizing the need to lay down our hearts before Jesus, allowing him access into our inmost being. This is why, even if you don’t have branches of any sort for your celebration, you can still participate in the spiritual theme of Palm Sunday. 
 
Why does the priest wear red? 
Red is the color of blood and symbolizes love, fire, passion, and the blood of sacrifice. Red is worn on Palm Sunday, Good Friday, any day related to Jesus’ Passion, on Pentecost and on the feast days of those who died for the faith (martyrs). 
 
Why are statues and images veiled? 
It seems strange that during the most sacred time of year Catholics cover everything that is beautiful in their churches, even the crucifix. Shouldn’t we be looking at the painful scene at Calvary while we listen to the Passion narrative on Palm Sunday? While it may appear counterintuitive to veil statues and images during the final weeks of Lent, the Catholic Church recommends this practice to heighten our senses and build within us a longing for Easter Sunday. It is a tradition that should not only be carried out in our local parish, but can also be a fruitful activity for the "domestic church" to practice. 
 
Why is the Gospel reading so long? 
For Roman Catholics, the weekly Sunday Mass readings are extremely short when compared to the Passion narrative that is recited (or sung) every year on Palm Sunday. This makes Palm Sunday somewhat difficult to attend for those who have trouble standing for the entire Gospel proclamation. However, did you know that every Sunday may have been like that in the early Church? Many of the first Christians were Jewish, and so, not surprisingly, they modeled their liturgy on the synagogue services. This included a continuous reading of Sacred Scripture that was carried on from one week to the next. It was divided up into two separate readings, one from the "Law" and the other from the "Prophets." 
 
Why does the congregation participate in the Passion reading? 
Holy Week is the most sacred time in the Church’s liturgical calendar, entirely focused on Jesus’ Passion, death, and resurrection. A recurrent theme throughout the week is a call to accompany Jesus during this most painful part of his life on earth. Palm Sunday opens Holy Week with a solemn recitation of Jesus’ Passion, and typically this involves each person having a role. When celebrated in a church, the parishioners often take the role of the crowd. This culminates in the entire congregation saying, "Crucify him! Crucify him!" In this instance we recognize the role our sins have in Jesus’ crucifixion and how Jesus suffered and died for us, even though we weren’t even born yet. 
 
What should I do with my blessed palms? 
After leaving church on Sunday, you may have come home with several long palm branches from the celebration of Jesus’ triumphant entry into Jerusalem. You may be asking yourself, "Well, what should I do with these?" Whatever you do, don’t throw them away! According to the Code of Canon Law, blessed items are not to be discarded in a trash can, but treated with respect (cf. #1171). At Mass these palm branches were set apart by a blessing from the priest and made into a "sacramental," an object that is meant to draw us closer to the celebration of the seven sacraments. Throwing them in the trash ignores their sacred purpose and treats them like any other object we no longer need.

No. The USCCB clarified this in September 2012 that it is no longer permissible to alter the text of the Lamb of God. The number of times it is sung may be lengthened however to cover the action. 
 
The Agnus Dei is meant to be sung during the breaking of the host. The sign of peace, which should be brief, ends and then the Agnus Dei is sung. It remains problematic that the sign of peace is often difficult to end since many treat it as a kind of meet and greet rather than a quick sign of charity to those immediately nearby. Catechesis is necessary to keep the sign of peace brief.

There is an understandable concern today about the way many people dress for Mass. There are double issues of modesty and also of people attending in clothes that seem far too casual for the holiness of the Mass and of God’s house. 
 
The problem begins as a cultural one. The fact is Americans seldom dress up any more for anything. Even many work places that once featured uniforms and/or suits and dresses, have become very casual. Modesty too is a cultural problem that includes clothes that are often too tight or revealing. 
 
Now culture is very influential for most, often, sadly, more influential than faith. Deep faith would seem to inspire a devotion, and sense of the sacred for the Holy Liturgy and for God’s house. But due to poor formation in many, the influence of culture prevails and most think little of how they dress when going to Holy Mass. Frankly most do not intend any irreverence, but simply dress without a lot of thought. 
 
Thus, a problem in issuing a dress code is that there is a range of acceptable views on clothing. In fact, the word “modesty” comes from the word “mode” referring to the middle of some range of views. And frankly, standards vary across time and cultures and especially regarding age. I have often found that many younger people are surprised to hear that what they wear might be considered irreverent and express a little irritation. Older folks remember different times when standards were different. 
 
That said, a general norm for men might be: trousers, not jeans, a button-down shirt, or at least a T‐shirt with a collar, no crazy slogans. For women, a skirt and blouse or dress at knee level or lower. Women should avoid low cut blouses. Sleeveless blouses are debatable. But in saying this, it would generate a lot of very different opinions. 
 
Hence, perhaps the best we can do is to gently remind all people of the sacredness of the Holy Mass and seek to grow their faith in how special the Mass is. As for modesty, more significant moral issues are involved, but so are greater sensitivities. It is a very delicate matter for a priest to speak in great detail about women’s fashions. Frankly we wish older women would take the lead here, and speak to younger women. Priests and men can speak to younger men, but here too, laymen ought to lead in this manner.

For as long as religious dietary guidelines have existed, somewhere there has likely been at least one moderately devoted practitioner desperately searching for loopholes. 
 
But the advent of technology that enables non-meat products to taste more like meat than ever poses a fresh ethical question that’s particularly relevant this time of year: Can Catholics, in good conscience, eat plant-based meat substitutes like the Impossible Burger during Lent? 
 
“I will be honest: when someone asked me that, my first thought was, ‘Why didn’t I think of that?! It’s genius!!’ ” the Rev. Marlon Mendieta, of St. Patrick Catholic Church in Fayetteville, N.C., wrote in an email. “But then my conscience kicked in, and I just felt that I wouldn’t be okay with that.” 
 
The Catholic Church instructs members to abstain from meat on Ash Wednesday and Fridays during Lent, a season of penitence and renewal leading up to Easter. The practice of forgoing meat dates to the early Church, when meat was considered a luxury, and is meant to be an act of self-discipline. 
 
To Mendieta, whether a Catholic should pass over the Impossible or Beyond burgers in the grocery store in the spirit of self-control is “a huge question mark.” So he said he took an informal poll of his priest friends. 
 
“If it’s not meat, it’s not meat,” one friend reportedly responded, ushering in a sigh of relief from Catholic omnivores everywhere. 
 
“Seems like it goes against the spirit of the penitential season if we just eat things that taste like the stuff we’re supposed to be abstaining from,” said another priest, putting the kibosh on meat eaters’ momentary joy. 
 
Ultimately, Mendieta said the decision is a matter of personal conscience. Plant-based burgers are not meat, he said, but eating them because they offer a technical exemption to the rule is kind of missing the point — unless there’s a medical reason, like an allergy to fish, which Catholics commonly substitute for meat during Lent. 
 
Still, the Church generally cares more about whether eating a certain food is a sacrifice than about whether it’s technically meat, said Michael Foley, a patristics professor at Baylor University. Church leaders let Catholics in parts of Michigan eat muskrat, for example, because it’s considered less desirable than other animals. On the other hand, Foley said St. Augustine of Hippo used to criticize wealthy fourth-century believers who on Fridays during Lent ate extravagant seafood, which does not count as meat. 
 
Abstaining from meat is also meant to be tied to almsgiving, or donating to charity, Foley said. Catholics are supposed to give to people in need whatever money they save by not buying meat during Lent. If a plant-based burger costs as much as or more than a burger made from meat, Foley said, the charitable part of the tradition is missing. 
 
A year-round vegetarian, the Rev. Christopher Steck sometimes eats plant-based meat substitutes along with other types of veggie burgers. But Steck, a theology professor at Georgetown University, said he often substitutes peanut butter sandwiches on Fridays during Lent because he considers the meal’s simplicity to be a sacrifice. 
 
Although Steck said eating plant-based burgers could break with the spirit of abstinence from meat, he added that Catholics could justify that choice if they made it for reasons of sustainability. Pope Francis has appealed to all people to care for the environment and protect the Earth’s future. 
 
“If that’s their motivation for a plant-based burger,” Steck said, “to me, there’s a nice symbol in that.” 
 
For those hoping to replicate the taste of meat this Lent, David Cloutier, a moral theology professor at Catholic University, offered a resolutely uplifting perspective: “They’re 100 percent in the clear.” Catholics should ask themselves whether eating a plant-based burger represents a sacrifice for them, Cloutier said. Even if it doesn’t, he said it still adheres to the letter of the law, which is meant to remind Catholics that it is Lent. 
 
But if some day scientists start growing burgers from animal cells in a laboratory, Cloutier said Catholics will have to redefine “meat” all over again.

In the scriptures, the title "Son of God" is used in many different senses and is, paradoxically, more vague than the title “Son of Man.” “Son of God” can be a title of Israel itself (Ex. 4:22; Hos 11:1), of the Davidic King (Ps 2:7) and of the angels (Gen 6:2), all humankind, all the just and peacemakers are called sons of God (Mat 5:9), and so forth. 
 
In view of the ambiguity of the term, this is why Jesus did not simply say, "I am the Son of God." Rather, he spoke more clearly, saying for example, “The Father and I are one… to see me is to have seen the Father,” etc. Indeed, the anger and charges of blasphemy by many of the Jews at Jesus' time show that Jesus’ claim to divinity was far better accomplished this way than to us a more ambiguous term of that time: “Son of God.” 
 
Paradoxically, “Son of Man” is a clearer profession of divine transcendence that can be traced to Daniel 7:13 which Christ appropriated to himself. That prophecy speaks of one, like a Son of Man coming on the clouds to judge the earth, who has a Kingdom that shall never end. 
 
Jesus’ preference for the term is shown when Caiaphas the High Priest said: "I put you on oath by the living God to tell us if you are the Messiah, the Son of God." Jesus answered, “The words are your own. Moreover, I tell you that from this time onward you will see the Son of Man seated at the right hand of the power and coming on the clouds of heaven." (Mt 26:62-66) 
 
Thus, "Son of Man" is a more clear and lofty title, which Christ prefers for himself.

The terms “soul” and “spirit” are often used interchangeably in modern English, and also to some extent in the Scriptures. They are synonymous, in the sense that they are not describing two separate realities. The human spirit is not some third part of the human person, separate from the soul. Rather, as an aspect of the soul, the human spirit (as distinct from the Holy Spirit), is that aspect of our soul that opens us to God. Some theologians speak of this openness of our spirit as giving us capax Dei (a capacity for God). That is to say, since our souls are spiritual and rational, we have the capacity to know and interact with God. And thus, the spirit is that aspect of our soul which most distinguishes us from the animals. 
 
In this distinction of soul and spirit, the Catechism says the following: Sometimes the soul is distinguished from the spirit: St. Paul for instance prays that God may sanctify his people "wholly" with "spirit and soul and body" kept sound and blameless at the Lord's coming (cf 1 Thess 5:23). The church teaches that this distinction does not introduce a duality into the soul. "Spirit" signifies that from creation man is ordered to a supernatural end, and that his soul can gratuitously be raised beyond all it deserves to communion with God. (Catechism #367).

Generally, the penance should be performed on Friday, though exceptions can be made due to other obligations such as attending family or civic celebrations. Strictly speaking, one can work out deferrals or dispensations in regard to Friday observances with their pastor, but practically, most simply work through this on their own. 
 
The thinking back in the 1970s when “meatless Fridays” were substituted with a day of penance was to offer other observances to people on Friday. Simply giving up meat and going to Red Lobster was hardly a penance for most, though the law was being observed technically. Hence it was thought to permit any range of penances, from giving up other things, to taking on special prayers or works of charity. 
 
But as your question implies, it is difficult to follow an uncertain trumpet, and many Catholics simply drifted from any Friday observance with such wide-open parameters. Psychologically it would seem that having a clear focus is necessary to assist in such practices. Hence, some Bishop’s conferences are going back to meatless Fridays. 
 
Here in America, that is not the case, though there has been some discussion. For now, you are largely free to determine how to observe Friday, presuming it has a penitential character. It could be to abstain from something good, or to take on some pious or charitable work.

Midnight Mass on Christmas is an old tradition, based on the belief that Christ was born at midnight.  We really don't know the time of his birth, but the origins of this tradition may lie in a passage from Wisdom (18:14-15): 

For when peaceful stillness compassed everything and the night in its swift course was half spent, your all-powerful word leaped from heaven . . . 

In the early Church, midnight was not assigned as the precise time for the first Christmas Mass.  It was simply to be celebrated "during the night."  Later regulations said it was to be "ad galli cantum" ("when the rooster crows"), which was more towards 3 AM.  Spanish-speaking people still refer to the midnight or early morning Christmas Mass as the "Misa de Gallo" ("Mass of the Rooster"). 

Since about the end of the first millennium, the custom developed of celebrating three Masses, beginning at midnight. 

The first was called the "Mass of the Angels" because the Gospel was from Luke's account of the angel's announcement to the shepherds. 

The second was called the "Shepherd's Mass" because the Gospel was the account of the shepherds' visit to the manger. 

The third was called the "Mass of the Divine Word" because the Gospel was from the prologue of John -- "And the Word became flesh . . ." 


SOURCE: The Little Blue Book: Advent and Christmas Seasons 2019-2020, 12/24/2019.

The simple answer to your question would be no. The targeted nation could not simply retaliate in kind, and certainly not seek to obliterate the offending nation. 
 
Some sort of military response might well be called for, even full-scale war. This would presume that the criteria for just war have been met. Namely, that the damage and threat was grave, lasting and certain (which would be the case in your scenario), that other means of ending the conflict has been tried or were not possible, that there is a reasonable hope of success in turning back the threat by military means, and the use of arms must not produce evils and disorders graver than the evil to be eliminated (cf Catechism #2309). 
 
Even once within a war, a nation must use means to distinguish between combatants and noncombatants. Hence, choosing to wipe out whole population centers, indiscriminately killing combatants and noncombatants, is not a moral option. Indiscriminate obliteration cannot be condoned under any circumstances. (cf Catechism #2314). 
 
It will be admitted, given the existence of large-scale nuclear devices, the most effective means to deter such attacks is complex and debatable. A short answer such as this cannot possibly explore all the points debated in the deterrence of nuclear threat. Simply here let it be noted that the Catechism expresses strong moral reservations regarding the modern “arms race” (cf Catechism #2315). 
 
Thus, to return to the main question, if a nation were lamentably to be attacked by a country in the way you describe, that nation is not thereby justified in indiscriminately retaliating by wiping out whole cities or in annihilating that country.