Orwell wrote:
I've had ancestors who fought in the Revolution, in the Civil War, and in the Second World War. I appreciate and honor the sacrifices people made in the past to establish, preserve, and defend this country. I do not support people who contribute to wars of imperialism (World War One, Mexican-American War, Spanish-American War) or pointless and wasteful conflicts (War of 1812, Vietnam, Iraq).
I think this is a more important reason than the motivations of those who join the military. Once you've joined, you have a lot less choice over what you take part in, and if your country has a history of interventions that would be defensive only in Bush's sense of preempting any possible threat not only to the country but also to the profits of the companies funding the politicians, then reflexively supporting the troops is morally dubious. At least, if by "supporting the troops" you mean supporting whatever they've been told to do.
For those who think I denigrate the integrity of the US political system without cause, I recommend reading Gore Vidal's account of the US intervention in Guatemala in 1954, included in his book
Dreaming War.
If you think the intervention in Guatemala (and the following four decades of civil war costing about 200000 lives, mostly at the hands of government supported death squads and the army) was a one off, work through
this list of military interventions and see how often the intervention was a response to a serious threat. Add the wars against the native inhabitants of the continent, and see how many of the treaties were broken by the US government whenever it was convenient.
Chesterton, in A Defence of Patriotism wrote:
"'My country, right or wrong,' is a thing that no patriot would think of saying. It is like saying, 'My mother, drunk or sober.'"