"Only one thing can conquer war: that liberal attitude of mind which can see nothing in war but destruction and annihilation, and which can never wish to bring about a war, because it regards war as injurious even to the victors." - Ludwig Von Mises
The central axiom of libertarianism is that noone may aggress against the person or property of others; put more fully, each individual may act as they choose so long as they do not enact aggression on the person and property of others. It would therefore be the case that war is the biggest violation of libertarian ethics that there can possibily be. While murder and vandalism are small-scale violations of liberty that are obviously opposed, war is an engagement that multiplies this violation to huge masses of people and property. War is the mass-destruction of person and property: mass-murder and mass-vandalism. War is "legal murder" on quite a large scale. It therefore logically follows that the libertarian must be staunchly anti-war.
This does not mean that the libertarian must be a total pacifist (although they can be, but most aren't), because self-defense is a value that comes about as a result of non-aggression. Self-defense is not an overt act of aggression, but a mechanism against and in reaction to it. The people who are defending themselves from acts of aggression are entirely justified in their defensive action. It nonetheless remains to be the case that defense is most often the excuse used to justify aggressive war. Of course, this is a disingenuous notion; a lame excuse for mass-murder.
Is it possible to have a "just war"? It depends on how we define war. If by war we mean an act of aggression and invasion of a territory, then there cannot possibly be such thing as a just war. The very nature of such an act is offensive, not defensive. However, if by a "just war" we mean the act of defending oneself against such an act of aggression, then this defensive action indeed can be justified. But to call such defensive action war seems to be innacurate. War would be the act of invading and aggressing. The people defending are not invading or aggressing. It is upon this basis that we justify those who take action against an invading aggressor.
To forbid such self-defense would be a gigantic error that would, in practise, function to allow aggressors to get away with their acts while their victims sit idly by while being plundered. Surely we would not want to forbid people from defending themselves. On these grounds, technically the "insurgents" in Iraq are justified so long as they are only defending themselves against the invading and aggressing force that occupies them (those who do more than this, who blow up people randomly, obviously are not justified on these grounds and do not qualify as insurgents).
Most wars come about as the result of a conflict not between two countries, but between two or more governments. This conflict is often territorial, although it also may be economic. Wars are often waged by a government in the attempt to economically or politically dominate a particular area outside of its periphery. In short, when a government wishes to expand its territory, it resorts to war. And when a government wishes to exercise authority over areas outside of its territory, it resorts to war. Most war, therefore, is a consequence of the nature of nation-states in themselves. It is practically inevitable that there is going to be conflict between the jurisdictions of two given nation-states because nation-states have a natural tendency to expand and centralize. There is no easier way for a government to achieve this than through war.
In many ways, in earlier times it was obvious to most people that wars were a conflict between two governments, two sets of rulers, as opposed to a literal battle between peoples in themselves. However, this has been increasingly blurred over time by the false idea that wars are fought to defend the people, as opposed to political interests. Essentially, the difference between nation and state has been expressly blurred to justify war. The move towards modern warfare has effectively made people even more willing to genuinely support them and enlist themselves to fight in them then they would have been in centuries past.
What motive may governments have to engage in a war? War presents governments with the oppurtunity to recieve the obedience of the domestic populace, by putting forth the notion that the war being fought is in defense of them (when in reality it is in persuit of the political interests of the ruling elite and those who ally with them). War also presents governments with the oppurtunity to increase its police and executive powers domestically, and gain support for any other policies it may promote in war-time. Further, war presents governments with the economic oppurtunity to raise more revenue, particularly through the method of increasing the money supply.
With these economic oppurtunities, governments provide an incentive to effectively "buy" allies in the private sector - that is, it presents governments with a smooth means to ally at the hip with big buisiness. Another motive that governments have to engage in war is to set up a means to politically and economically control the region being invaded; to indirectly control foreign governments and natural resources. In other words, empire and plunder. While wars have been fought in the name of religion and other such things, these rationales are more often than not only a cover for the real political motives involved.
Propaganda is an essential tool in order for the political class to dupe the population into supporting a war. The purpose of war-time propaganda is to convince the people that there is a constant enemy that threatens their lives (I have debated with quite a number of American right-wingers who literally believe that we face the threat of Muslims invading our whole country and imposing a Muslim theocracy onto us). If no enemy exists, one is concocted out of thin air. The people are to be given the impression that the war is a preservation of their "traditions" and values. Whatever general political, ethical or religious values people have are exploited as to give the war an appearance of nobility.
Interestingly, many modern wars are propagandized for with arguements claiming that the war serves egalitarian notions such as democracy and equality. The war propgandist's job is essentially to convince everyone that "slavery is freedom", that war is justified because it "protects our freedom". This line of arguement is a reducto ad absurdum, of course, because you cannot protect freedom by violating it. The idea that you have to give up freedom in order to preserve it is purely Orwellian. It belongs in the same "dangerous ideas" bin as fascism and communism.
The way in which war effects the liberties of the domestic population of the war-waging country itself is devastating. Much of this impact is economic. It would be impossible to fund a modern war completely with taxes. Further, such levels of taxation would be direct and obvious to the public, and therefore it would conjure considerable resistance. This is why goverments rely much more on other means to fund wars: by borrowing money from foreign governments and creditors, and by increasing the money supply itself. The borrowing brings about debt, the cost of which is passed onto the public. The increasing of the money supply brings about inflation, which manifests itself by a rise in the cost of living. As such, war brings about conditions that considerably decrease the economic well-being of the population of the war-waging country.
Economic liberty is violated further by the formation of what we may call "the military-industrial complex" - the alliance between the military establishment and various industries and buisinesses. The word industrial is in there for a reason. This alliance presents a means for big buisinesses to get special privileges; tax-funded contracts, bail-outs, and assorted special protections from competition. All of this is at the expense of the general population in the form of inflation and taxes. It constitutes a wealth transfer to various rich corporate elites. What would be considered the "working" and "middle" classes pay for this the most.
War also presents many threats to the personal liberties of the population of the war-waging country. If there is a draft, then this means that various people will be conscripted against their will to accept and keep a job, and this is a job that poses a greatly heightened risk of injuring or killing them. A draft is a violation of voluntary contract, plain and simple - it is tantamount to slavery. Slavery means that you are forced with the threat of violence to take a job and to keep that job. It's involuntary servitude - a coerced contract; slavery to the state. Further, during war-time, dissidents face the possibility of being thrown in jail or even violently aggressed against by law enforcement (see the Kent State shootings).
Expansions of police powers during war-time provide a means for the government to effectively nullify precedents that are meant to protect the citezen such as habeus corpus, the requirement of a warrant and reasonable cause, freedom of the press, freedom of association and any real sense of "personal privacy". In short, war provides a chance for the political establishment to break the law. The expansions of political power that take place during war never shrink back to where things were before. The post-war nation is never the same as the pre-war nation. It is always in a situation with more political power, and therefore less individual liberty on the part of the population.
While wars most certainly have been going on since far back in history, the 20th century marks the rise of total war. The 20th century marks the rise of war-technology capable of mass-destruction at the push of a button; the atomic bomb presented us with a means to destroy an entire city with a weapon the size of a human fist. In comparison, as bad as the past may have been in many respects, the 20th century makes the middle ages appear to be peaceful times. The 20th century was in a state of non-stop war in many ways, and now that we have moved into the 21st century this hasn't stopped.
It was only in the 20th century that we could even concieve of the advent of a world war, and now we have had two of them so far, and we might be moving towards a third. Sadly, international treaties and bodies that are supposed to be dedicated to global peace have done almost nothing to stop the advance of war (and in some ways, they may have advanced us towards war in themselves). A policy of foreign intervention is inevitably a pretext for war. Only a policy of non-intervention and free global association can prevent war.
There is no more pressing of an issue in our times than war. The establishment of peace should be the main goal of humanity. People need to engage in a strong advocacy and reassertion of the principle that it is peaceful cooperation that is the boon of civilization. Aggression is the destructor of social order.