Since World War II, the anti-war movement in America has been almost exclusively partisan, saying that the United States was supporting the wrong side or was itself the aggressor. In some extreme cases, adherents to the anti-war ideology have even gone as far as to falsely label anyone who even has a military at all, let alone go to war and/or have significant military buildup, as fascists in a false equivalence to the Nazis during World War II (usually also with the similarly false association of Nazism with the right wing and conservatism).
Thus the term anti-war is often used specifically to indicate political opposition to America's war aims, notably during the Vietnam War and the invasion of Iraq. While reminiscent of pacifism, the "anti-war" movement is not opposed to war per se, but only America's involvement in it.
Often overlooked by anti-war activists is that the American military is volunteer today, and service is not compulsory.
So-called "anti-war activists" are content to let foreign governments invade other countries and slaughter or enslave innocent people, as long as the United States isn't involved. Their reasoning, which is more often assumed or implied than stated explicitly, is that the United States is imperialistic to the core, and that no American military intervention could possibly be a good one (see "American Empire").
The term anti-war on the surface would seem to be an opposition to war itself (see pacifism), or similarly, the wish that a specific war would simply end immediately (regardless of outcome). "Tenting Tonight on the Old Campground", Civil War ballad by Walter Kittridge, is a song with no reference to victory but only a wish for "the dawn of peace". 
Partisans frequently exploit this confusion, using terms such as peace and anti-war to imply that there's nothing worth fighting over with such violence, while downplaying or hiding their desire for one side to win.
Phrases like "opposition to the war" likewise are not meant as an even-handed condemnation of both sides (for violating pacifism). Rather, it was used as criticism towards one side's participation in the war which is being opposed. During the Vietnam war "anti-war" protesters would even chant slogans like "Ho, Ho, Ho Chi Minh! The NLF is gonna win!"
Even during World War II, there was anti-war during the first 26 months of the war within America, mostly due to the Molotov-Ribbentrop pact between Nazi Germany and the Soviet Union that had occurred during that time. However, as soon as Operation Barbarossa happened, the so-called "anti-war" protestors eagerly supported American involvement in World War II overnight, evidentially due to their support for Stalin.