History teacher here. I'm about to start teaching World War 1 for the first time and I'm planning on teaching my students on the Rape of Belgium. I get that the name Rape of Belgium is partially anti German propaganda, but it got me thinking about how common it is for conquering armies to commit rape in warfare. Is this something that both sides do rather consistently or is it more dependent on the specific factions and the conflict?
Rape has been legally (under international law) considered a war crime since after World War II because (a) it is so horribly common and (b) it is a tactic for terrorizing civilians, not just a couple of one-off men being evil. If you're interested in the politics of how this came about (and why it took countries so much longer to ban rape as a war crime than looting, despite objections to both being initially presented at the same time), Tuba Inal's book Looting and Rape in Wartime: Law and Change in International Relations should interest you.
Otherwise, we have a few earliers answers on AskHistorians that should begin to illustrate how wretchedly common rape as a war crime is:
That's just a few different contexts. But they are divergent enough at least in time to suggest what a devastating and ongoing presence rape has had and has in warfare.