Tbh I think people are trying to hard to dunk on you rather than actually explaining how we see things and why.

Opposing war is generally the correct take, in most cases, but that doesn't necessarily mean that you should turn it into a hard rule, because there are exceptions. The American Civil War is an example I think most people would agree with. As violent and bloody as it was, it was still outweighed by the centuries of systemic violence baked into the system.

As Marxists we concern ourselves less with "who started it" (an inherently subjective question) and more with who's fighting it and why, and what outcomes can be expected. War is the continuation of politics by other means, so to understand a conflict it's important to look at the political questions at stake, on a case-by-case basis.

Without getting into the specifics of these conflicts, that's what's meant by "anti-war-ism," not just opposing war, but doing so without really bothering to understand the specifics of a given conflict.