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). In addition, owing to how liberals misdefine freedom several times and give a false choice between freedom and fascism, they also often imply that America going to war at all is a threat to freedom, even though events such as the founding of America during the American War of Independence had been accomplished via war, and often ignore that events such as the rise of Nazi Germany and the French Revolution had been the result of internal squabbling by like-minded left-wing political factions as well as trying to forcibly establish utopia, or that the Cold War had been started due to overt Communist aggression in Korea as well as Soviet expansionism, among others, or that the advancement of the welfare state was far closer to a threat to American freedom than going to war.
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 conveniently 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 - most often Communist or Islamic dictatorships - 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"). In some ways, it's also comparable with anti-Americanism.
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 the participation of one side in the war (invariably, America) 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!" In fact, "anti-war" movements from World War II onward, most notably during the Vietnam War era, most often were little more than fronts for Communist groups being directed and supported directly from the USSR; those movements, populated by naïve and easily influenced youth being used as useful idiots by the movements' leaders and supported by the liberal media, sought the disarmament of the United States and its allies, knowing all the while that the USSR and other Communist dictatorships had no intention of disarming themselves, and those movements were willing to use rioting and other violent tactics and criminal behavior (such as during the Kent State protests) to achieve their aims (which, in fact, was goal #19 of the 1963 Communist goals for America), while the USSR and allied countries that gave those movements covert support continued their aggressions against the free world.
Even during World War II, there were anti-war movements in operation 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, which saw CPUSA-backed left-wing protestors involved with those movements actively supporting both Nazi Germany and the Communist USSR while undermining American support for the United Kingdom and other countries under Nazi attack, while in England, Communists in that country aided covertly in the Nazi attacks on their own country by spying on the British military and sending information on them back to the Kremlin in Moscow, which would in turn relay the information to Berlin. However, as soon as Operation Barbarossa happened, the so-called "anti-war" protestors eagerly supported American involvement in World War II overnight, evidently due to their support for Stalin. In some cases, such as Howard Zinn, they even directly participated in the war by enlisting (though not due to any patriotism to America or Western Europe so much as to aid the USSR). Ironically, the Korean War had minimal anti-war influences, largely due to the Communists being embattled by Joseph McCarthy and HUAC's investigations.
- The Vietnam Anti-War Movement: A Study of Communist Tactics In America (link dead, go here for archived page)
- Vietnam War, Part II – The "Peace Movement"
- AIM Report: Communists Run Anti-War Movement
- 19 - Use student riots to foment public protests against programs or organizations which are under Communist attack.
- Why No "Peace" Movement in WWII?