I would agree that, most of the time, war isn't good for the collective, although it might depend on the war. The Civil War, for example, although that can be complicated because the States were smaller collectives within the larger collective of the USA as a whole. Though there were other collectives defined as "black" and "white" (among other identities), but on the other hand, there were many who wanted all inhabitants to be part of a singular American collective, where all would be citizens with rights regardless of their race or national origins. Constitutionally, that goal was achieved by the passage of the Fourteenth Amendment, so one might say that the Civil War benefited the American collective by making it larger and more solidified.
Of course, that's not to say that the wealthy classes didn't grab the lion's share of the spoils and used their political power for their own benefit. Even at the lower levels of the hierarchy, some people were able to go west and take part of the great land grab, about the same time the Europeans were scrambling for Africa. Nationalism is also collectivist, albeit limited in its scope, which favors their own collective over other collectives. Capitalists went along with that because nationalism is an effective motivator and tool to gain recruits, who join up out of patriotism and love of country, while the capitalists collect all the spoils. It's a sweet deal, as long as you're on the winning side.
But, as was the case with WW1, it can leave a bad taste in people's mouths, even those of the victors. The slaughter and atrocities had gone too far, and at least in terms of the health and well-being of the collective as a whole, it had a negative effect on the psychology and self-perception the collective had. It's reflected in culture, policies, and political shifts where we started to question ourselves and what we were doing more and more. Maybe a slow awakening that peaked in the decades following WW2, though we seem to be drifting back into somnambulance in more recent times.