I think it's complicated. I don't think Butler believed that every single war was unjustified. When reading 'War is a Racket', you have to look at the context of war in which we lived. He was too young to fight in the American Civil War. So his consistent experiences of war were those fought in America's most blatantly Imperial phase. Even compared to Vietnam, Iraq, etc, the wars of the turn of the century were cravenly driven by capital and imperial ambitions.

And to that matter, some of what brought the U.S. into the orbit of WWII was a lower key version of the kind of foreign policies of that earlier era. Disrupting Japanese interests in Asia was very much about economic benefit and power balance in the mode of realpolitik.

And in the end, the Roosevelt administration remains unique in American history for how hard it pushed against colonialism. Truman was much more aligned with British interests in this way than FDR. So while the war is justified in the name of stopping two monstrous regimes, its precursors and its aftermath do carry a fair amount of the same taint that Butler thoroughly observed in his time.