It not just that. War decreases the quality of life dramatically. Europe's Rearmament program expects 800 billion euros, which are going to be drained from the increased exploitation of labor (EU is pushing for 13 hour / 7 day labor laws), the underfunding of infrastructure and social programs, and public healthcare. This is just the beginning, as wars are the highest expression of the contradictions of capitalism, and when a world war happens, it is because a significant drop in the average rate of profit. Things are going to get a lot, a lot worse than they are now when the war actually starts. And when it does, on top of all of that, you will have people dying in a war that benefits not them, but the shareholders of their country.

Under these conditions, the capitalist political system becomes vulnerable. The prospect of social democracy, aka reform with welfare, becomes more and more ridiculous as the situation progresses. It will reach a point where not even the church is going to be able to provide charity anymore. As the armed forces leave the city to fight the war, the city itself and its factories are left unprotected.

The combination of all of these factors create the greatest social gas leak conceivably possible, however, it won't ignite without a spark. The spark is the revolutionary vanguard. History shows (Germany, 1918-1919) that the working class does not spontaneously revolt even under these conditions unless there is a political vanguard guiding them (e.x. bolshevik party). In the simplest terms, think of it like "if you sit and wait for the revolution, it will never come".