Nationalists will so long as they can wrap it into their logic of domination. That is surely why Modi et al thought they could start an all-out war.

Organizations vary massively by region, but if you oppose this war I recommend getting plugged in locally, especially if there can be any kind of tie-in with the May 20 planned strikes. That is probably the most powerful action that could be taken against war, though not at all an easy one.

As always, don't reveal any personal info online, e.g. telling me/others your locale! Stay safe and organize carefully.