PwnallthethingsBlowing up dams--even if the dam is a military object--is a specifically enumerated war crime under the Geneva Conventions, if attacking that object would cause the release of dangerous forces (specifically: flooding) that attacks or could cause severe losses to the civilian population
Bater Kari@Pwnallthethings In The famous Dam Busters raid of WW II, numerous people were killed by flooding. Many were forced laborers used by the Nazis.
The Geneva Convention was updated in 1977 making dam-busting a war crime. The Geneva Convention does contain an exception for certain military activities.