Consider that there was only one culture on earth that had no word for, and indeed no concept of, war. That was the Aleut (Eskimo) culture prior to first contact with the outside world. The reason for this was economic. In that culture, if a group of guys got together, they immediately organized a hunting party or starved to death. There was never enough surplus to support large-scale group violence.
In every other culture, there have been enough resources for groups of guys to fight each other, without having to spend all their time trying to feed themselves. Hence, war is a phenomenon of wealth, or at least of something other than crushing poverty.
By the same token, war can be said to be caused by freedom. When Marshal Tito (backed by the Soviet Union) ruled Yugoslavia, his police state apparatus kept its boot on the necks of the Serbs and Croats so that they couldn't fight each other. Only after Tito died and the Soviet Union collapsed, did people have enough freedom to rabble-rouse, obtain weapons, and begin the recent civil war. (Sort of like India's present Bal Thackeray whose party orchestrates anti-Muslim violence in India to gain power at the ballot box.)
It is, of course, wicked to advocate forcing all humanity to endure starvation under a world-wide police state in order to eliminate war. Such a situation might be characterized by an absence of war, but it would not be peace.
Nevertheless, we can still salvage a useful idea here. The individual states comprising the United States do not fight each other, because the monopoly of violence is given to a higher power — the Federal Government. The individual states have given up their freedom to attack each other as well as their sovereign right to defend themselves. And this sacrifice is not just a paper committment: they have also given up the ability: states have no standing armies of their own. The National Guard may take orders from a state Governor, but not if the President of the United States commands otherwise.
In other words, world peace requires world government. Now, given that most of the world thinks the way to wealth is to take it from the United States rather than to generate their own, how soon do you think that will come to pass?
Moreover, peace requires a committment to deter, detain, and in some cases simply kill those would be warlords who bring disorder into failed and failing societies (because they want power and/or wealth, and see warlordism as the quickest way to get them). World government would not have the discernment to know when to act or against whom. Consider that Libya is on the UN Human Rights Commission, and that the United States may soon be removed. Consider that the UN took no action against Pol Pot in Kampuchea, or against the murderers in Rwanda. If the measure of effectiveness at securing world peace is body count, then it really is fair to call the UN an incompetent debating society.
So where does this leave us? Are we, after 2400 years of philosophical debate, back at "might makes right?" After all, what we call "Shock and Awe," the Germans once called "Blitzkrieg." With the US response to the 9/11 attacks in Afganistan, Iraq, some "law enforcement assistance" in Saudi Arabia and other Middle Eastern countries, and who knows what sort of action in regard to North Korea, we are witnessing what future historians may call, "The American Rampage."
On the other hand, the United States went along with the European powers who set up autocratic and semi-autocratic states throughout the world as the most convenient way to protect their economic interests in the colonial and post-colonial periods.
These states are now either transitioning to some form of democracy (perhaps not based on the New England township as de Toqueville described American democracy, but some form nevertheless) or they are becoming the so-called "failed" states that get taken over by warlords who can become international terrorists. Perhaps the United States is correct in (at least partially) abandoning its policy of assenting to what some writers call "autocracy lite" in Europe's former colonies, which assent was implicitly racist, historically unjust and ultimately counterproductive.
Perhaps, in order for the world to regain anything like a "moral compass" in international relations the world should look to the stated principles on which the United States was founded. To quote from the US Declaration of Independence:
When in the Course of human events, it becomes necessary for one people to dissolve the political bands which have connected them with another, and to assume among the powers of the earth, the separate and equal station to which the Laws of Nature and of Nature's God entitle them, a decent respect to the opinions of mankind requires that they should declare the causes which impel them to the separation.
We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness.—That to secure these rights, Governments are instituted among Men, deriving their just powers from the consent of the governed, —That whenever any Form of Government becomes destructive of these ends, it is the Right of the People to alter or to abolish it, and to institute new Government, laying its foundation on such principles and organizing its powers in such form, as to them shall seem most likely to effect their Safety and Happiness.
Herein we find grounds for the United States' effective redefinition of sovereignty — those governments are sovereign which derive their just powers from the consent of the governed. Those which don't are not entitled to have their claims to sovereignty respected. Those governments which cannot be regularly and nonviolently changed by the people they govern, are subject to violent overthrow from without, if they do not respect the most basic rights of their people and the peace of their neighbors.
In 1787, America was the only constitutional Republic in the world. By 1990, according to Francis Fukuyama's count there were 61. With the collapse of the Soviet Empire, there are even more now. After 216 years, the American Revolution is starting to catch on. Maybe it's time to give it a little help. Because the root causes of war are all traceable to governments that do not respond to, respect, and value people — their own or any other.