Depends on the situation. The Vietnamese used force to defend their country from us and finally kicked us out. This was necessary to their survival, and I don't have a problem with that. In Central America, various groups, guerrilla armies and states worked to lessen the impact of US violence, sometimes with force, sometimes through negotiations. The results were mixed, though, obviously, the US got the better of the deal, alas.
In the present moment, I really don't see the above mattering all that much. Al-Qaeda is more or less our creation, was used in earlier forms (under different names) to raise the stakes against the Soviets, and this was done for cynical, geopolitical reasons. Now, like Hamas attacking Israel (Hamas' former benefactor when the PLO began to moderate its stance), our little Frankenstein is now attacking us. And given its stated goals and primitive beliefs, I don't see a negotiated settlement in the offing. These fuckers have to go, but should be sent to their celestial reward in concert with a US effort to isolate them in the region. I think that the Bush admin's critique of Israel and lip service toward a Palestinian state should be seized upon by domestic groups and held to the fire. Instead, most simply chant "No War" and leave it at that.
When the US engaged the Third Reich, Jim Crow ruled the south, the military was segregated, abortion was illegal, Nicaragua had been raped by the Marines, and so on. But in my reading of that period I don't recall the left refusing to fight, citing these and many other sins committed by the US. They saw the immediate danger and acted accordingly. And they did not stop once that threat was vanquished.
DP