Poisoning wells falls into this category. But technology has definitely made industrial-scale devastation possible, starting with chemical defoliants. We've seen the use of depleted-uranium ammunition in the Gulf War and Kosovo. These armor-piercing shells vaporize on impact and leave radioactive residue. This is less an environmental concern than of indiscriminate weapons.
Cluster bombs detonate and spread hundreds, even thousands of bomblets.
Unexploded ones are left like landmines. They persist and make land uninhabitable after hostilities. NATO bombed this facility on eight or 10 occasions, creating gas clouds, which contained carcinogenic poisons. Dioxins were also released. This reportedly led to skin rashes, respiratory ailments, and concern about possible birth defects.