This will probably be a stupid question but I couldn't find any good info on the internet.
So, for a school project, I have to know if civilian casualties and the destruction of villages were crimes around the time of 1900. I'm pretty sure they were crimes, but I really need to know for sure.
Can someone help me with this?
It depends on the country, of course. For the U.S.: yes--go read a little about the Lieber Code.
The U.S. used it in 1898, in the Spanish-American War, and again in World War I. Germany used it in 1870, in the Franco-Prussian War. That seems to fit your time frame pretty nicely.
Good luck with your project!