I've never completely understood the Libertarian/isolationist fetish with war declaration; in its history the United States Congress has only ever declared war five times. Like it or not, the U.S. has a history of engaging in non-declared conflicts and interventionism of varying shades and degrees since it became a country. A good example of this is the Quasi-War with France from 1798-1800. Declared wars are not automatically just wars. The Spanish-American War is an example of a declared war that had questionably expansionist aims and made the U.S. an imperial power with its acquisition of Cuba, the Philippines, Puerto Rico, and Guam.
The Constitution says Congress can declare war, but it doesn't specify how this has to be done. Historically Congress has given assent to conflicts by either authorizing them, agreeing to fund them, or in the minority of cases, actually declaring them to be wars. The Civil War was the bloodiest and most costly in U.S. history, but it wasn't declared. Nor was Vietnam, which I agree was immensely wasteful, traumatic and unnecessary, though nonetheless funded by Congress. A case can be made for the Korean War, or Korean Intervention if you like, since there are now about 52 million free people living in the Southern democracy enjoying the benefits of good international relations and a market economy.
Libertarian foreign policy seems to imagine the world community as a domestic community writ large. Nations should act like sovereign self-respecting individuals with inherent rights and should only go to conflict in an agreed-upon manner for just causes. It'd be a nice world to live it, but the truth is the world has never, ever worked like that. The international arena is an anarchic system. "Just war" is a lofty enough idea, even when Aquinas was discussing it, but in the several centuries after Aquinas "just wars" were basically nobles large and small squabbling over each others territory, a handful of pointless crusades, and then later wars of religion and counter-revolution leading up to the unimaginable industrialized bloodbaths of the World Wars; a millennia of conflict finally ended by, largely, American intervention.
The main reason the world has achieved unprecedented advances in technology and commerce in the last few centuries is because a handful of powers exerted such influence that they could guarantee the security of sea lanes and give backbone to international law on the seas. Formerly and for a very long time this role was the pride of the British Empire and after WWII it was assumed by the U.S. Navy. At the close of WWII the United States found itself a global superpower and the only nation left standing with the capability to oppose the world's remaining totalitarian ideology of communism.
Lofty also the words of Mr. Jefferson, 'alliances with none', and all that. That's well and good when you're a marginal power on the fringes of the civilized world, but America's role and prominence in the modern (post-modern?) globe is vastly different than anything Jefferson could have imagined. Once the U.S. became a modern industrial power, the more difficult it became not to be involved with foreign affairs. Today the world is so instantaneously interconnected that pure non-intervention is impossible. If you agree to trade with someone and not someone else you will be accused of intervention. Is espionage intervention? Are sanctions? Are warnings to other nations? In a system where there are consequences to acting and not acting and you can be held to account for either scenario where does one really draw the line to what is and isn't intervention?
Simply put, until the U.S. can completely disentangle American interests and the economy from the global system it has spent decades shaping and maintaining it is going to find itself chasing or being sucked into foreign interventions.
