Why do people act like if you oppose a single unjust war, you must then oppose all war under any circumstances in order to maintain ideological consistency? 
It is inconsistent to oppose a war for reasons which are no less of a factor in a war you support, and unseemly to do so when motivated by partisan politics. If the reasons many opposed the war in 2003 were valid, they should be just as valid now.
These reasons include, but are not limited to:
- Wars are costly, and not worth the money and lives expended when not facing an existential threat
- Non-defensive wars are always unjustified (this would not be a defensive war in any normal sense of that term)
- Occupation in Muslim or Arab countries is doomed from the get-go
- The United States should not be the world's policeman
- The United States should not engage in nation-building (which would be morally obligatory if we were to invade Iraq and/or Syria. They would be just as unstable and vulnerable as they were pre-ISIS if we withdrew immediately)
- American intervention only, or at least always, creates blowback
- America's history of intervention gives us no moral authority to act as a mediator now
- We should not get involved in religious disputes (the intellectual and high-minded thing to do in the mid-2000's was to point out the history of Sunni-Shia conflict and claim that any attempt to involve ourselves in the dispute was futile)
If these things were true then, they are true now.
I had a hard time making up my mind during the first year or so of the Iraq War, and I'm conflicted about intervening with troops on the ground now, but I'm not going to change my view of the world and America's role in it because the guy in office has a D or R next to his name. My standard is this: a solid (not just reasonable or plausible) case must be made that we will impact the countries involved and the region for the better, that our homeland and citizens overseas will be more secure, that we have a strategy in place to provide for continued stability, and that it's worth the cost. A reasonable (far from solid) case was made in 2003 that intervention was in the best interests of us and the Iraqi people (mushroom cloud rhetoric aside), and that turned out to be a disaster. Hell, it was reasonable enough for most Democratic politicians to get on board. The burden of proof here must be much higher.
