In light of recent events in Europe, it’s a good idea to examine the principles of Just War theory in the Christian tradition. Just War theory is not specific to Christians, indeed Cicero of Ancient Rome is in many ways the father of the Just War tradition. That being said, major work on Just War theory has been done by Christians, and more specifically Catholic figures in the West, so it is to them we will mostly turn in this article. It must also be noted that the modern era has developed weapons and war time technology far beyond that which Augustine and Aquinas had any inkling of. How one wages war has fundamentally changed and that is its own discussion. Here, we’ll table that discussion and instead focus on the principles of Catholic Just War teaching.
Currently, the Roman Catholic Church lays out these conditions for a war to be considered legitimate, or just:
The damage inflicted by the aggressor on the nation or community of nations must be lasting, grave, and certain
All other means of putting an end to it must have been shown to be impractical or ineffective
There must be serious prospects of success
The use of arms must not produce evils and disorders graver than the evil to be eliminated
All together, these are the benchmarks, so to speak, of Just War. Underlying all of the benchmarks is the virtue of Justice which, as Cicero remarks in De Officiis, “is that no man should harm another unless he has been provoked by injustice. ” A person ought not harm another unless he has been harmed first. There are limits to this response “probably to the point where the man who did harm repents his injustice.” War, therefore, is the public response to unjust treatment from another nation or power. That does not mean, however, that war is always the correct response. Cicero, properly understood, makes it exceptionally clear that “a war can only be just if undertaken as a consequence of an unjust act. Thus, war of aggression  is always unjust.” Likewise, the Catholic Church clings to this principle to this day.
Another benchmark, the last resort criteria, can be found enumerated first in the writing of Cicero. He describes two types of conflict: “one proceeds by debate, the other by force.” The latter conflict, by force, should only be utilized as a last resort to punish injustice. In fact, if we understand Cicero correctly, “war is only permitted in order to restore a state of justice after an unjust act.” The principles of justice as communicated by Cicero can be understood as a duty not to inflict harm upon others and a duty to deflect injuries upon others. When applied to the question of Just War, as Cicero himself did, there is also a duty to protect others from injustice when capable. Thus, if one country or nation has been done an injustice, there is a duty to restore justice by war if necessary.
Other thinkers on the topic, such as Augustine, have either adopted the model of Cicero or used him as a bedrock for their own musings on the subject. Augustine, bishop of Hippo in the 4th century, is widely regarded as the father of Catholic Just War thinking. The extent to which Augustine formulated something new is up for debate, but the main idea behind his statement on wars is similar to that of Cicero: "We usually describe a just war as one that avenges wrongs.” Augustine’s position, as articulated by Ronald Kany throughout his essay in Just War to Modern Peace Ethics, closely mirrors that of Cicero. That being said, however, Augustine is quick to condemn war. Augustine is not in the business of making war and makes it explicit in his later works “the absolute deficiency of war.” A trend begins to emerge in Augustine’s later life wherein he begins to disdainfully look at the world. It can be said Augustine began to think “even the just war is nothing other than a sign of the imperfection of this world” and how it fails to imitate the next.
Thomas Aquinas, monk of the 13th century, taught and wrote extensively on many subjects, including this very idea of just war. His articulations follow Augustine and Cicero to an extent but he does add a novelty into the discussion. The first condition Aquinas lays out:
“In order for a war to be just, three things are necessary. First, the authority of the sovereign by whose command it is to be waged.” By this, Aquinas merely means it is not the duty of a private person to call together the people to wage war. Only the authority of the secular land has the right to wage war.
This benchmark, so far, has not been found explicitly in any real way by the other two thinkers discussed. Aquinas gives a quote from Augustine, but Ronald Kany gives a more robust account of the quote, and Cicero never addresses it. In current Catholic teaching, it is also absent. There is, however, an assumption of proper authority. The second condition would be impossible to be reached without a proper authority. Cicero, likewise, would most likely agree with Aquinas on this principle. The second principle of Aquinas, a war must be fought only in response to injustice, has already been covered.
The third necessary principle, according to Aquinas, is that of right intention.  This poses a few problems for modern readers. When does an intention make a war unjust? What is the right intention? 
Again, Aquinas introduces a novelty into the discussion. St. Thomas points to Augustine, but again the full breadth of Augustine’s teaching on the topic doesn't reveal this as a condition. It could be argued, however, that it is implicit in both Augustine and the current teaching of the Church.
While a foreign nation could be acting unjustly, if our goal is not to restore justice but get personal gain could it be really just to intervene. For example, if I saw a child getting bullied and only helped because the father might give me some money, would I really be acting justly? The act of stopping a bully might be just but the intention behind the act matters as well. Aquinas's right intention principle, while not explicit, heavily influences the Catholic Church's benchmarks.
The last two benchmarks, "reasonable chances of success" and "proportionality", are much harder to practically discern. By that, I only mean discussions around chances of success revolve around what "success" is defined as. If it's defined as the destruction of the opposition than it seems that we limit war too much as achieving such a goal is practically unreasonable in most cases. On the other hand, if success is nothing more than bloodying a nose than we give too much leeway to war as anyone can inflict minor damage. All the Church says is "The evaluation of these conditions for moral legitimacy belongs to the prudential judgement of those who have responsibility for the common good." 
A rather vague statement but so long as we opt not for either extreme, the benchmark is clear: if there is no chance at achieving military success, as that's what war is, then war cannot be just. Likewise, proportionality is tough to practically discern. Not so much because it's hard to respond well to injustice but most discussions about proportionality tend to discuss proportional actions in war. Our previous words on the subject make it clear that war can't be the first response to injustice and only in cases of serious injustices should it even be considered. For a more in depth approach to this part of the topic I recommend the Stanford Encyclopedia of Philosophy's article on War and in particular section 3.4 of their article.
The scope of this article avoided most of the discussion surrounding In Bello considerations for the sake of time. That being said, the Catholic Church makes it clear, as does her Tradition, that innocents are never to be the targets of violence. For further thoughts I again stress my recommendation of Stanford's Philosophy Encyclopedia discussion of the Just War topic as I think it addresses this very well and can give further reading suggestions in the areas that we didn't fully address here. Perhaps in a further article, especially if things don't alleviate on the world stage, we will continue a deeper analysis of the particulars within the aspects of what constitutes a just war.
Regardless if a war can be considered just or not, it must be stressed that war is not good. Like Augustine, we should be wary bellicose responses in the wake of international injustice. Not because we're scared of injustice but because it's yet another sign of our fallen nature. Most cultures, even Christian ones, have had a history of praising war or legitimizing it but war isn't something we should run towards. In the era of modern weaponry, more than ever, our considerations to wage war need to be weighed heavily. It is my personal opinion that total wars, a total mobilization of society to wage war, is unprecedented in the history of the world and something we're still wrestling with. Recent Catholic thought on the topic tends to limit the scope of Just War but in the era of carpet bombing, drone strikes, nuclear weapons, gas strikes, and the like, war has become more cruel than ever before and must not be taken lightly.
For the time being we should offer up our prayers for Europe and the conflicts raging therein.
St. Augustine and St. Thomas Aquinas, pray for us
Most Sacred Heart of Jesus, Have mercy on us
Cicero, De Officiis, 1, 20.
Andrea Keller, in Heinz-Gerhard Justenhoven’s From Just War to Modern Peace Ethics, 13.
By aggression, I mean wars of assault for honor, glory, or power. Defensive wars, when one nation unprovoked attacks another nation, are exercising a natural right to self defense. The decision to declare war, even if the other party has not done so, is not necessarily aggressive and is what we are discussing.
Cicero, De Officiis, 1,34.
Andrea Keller, in Heinz-Gerhard Justenhoven’s From Just War to Modern Peace Ethics, 14.
Found in Thomas Aquinas, Summa Theologica, II-II. 1, 40, 1, 83. Iusta autem bella ea definiri solent que ulciscuntur iniurias.
Ronald Kany, in Heniz-Gerhard Justenhoven’s Just War to Modern Peace Ethics, 45.
Thomas Aquinas, Summa Theologica, II-II, 40, 1.
For a more thorough understanding of this topic, see Gerhard Beestermoller in Just War to Modern Peace Ethics, 73-85. Far more complex than I stated above, it is an interesting read.
For a discussion on the limits of Aquina’s teachings as part of their greater historical context, James Muldoon in From Just War to Modern Peace Ethics has an essay that discusses the historical realities of medieval thinkers and how they both form the basis of our own understandings but also their limits.
Aquinas, Summa Theologica, II-II, 40, 1.
Both questions can be found and answered in Gerhard Beestermoller in Just War to Modern Peace Ethics, 89-95.
Catechism of the Catholic Church 2309