As you probably know, cesium is one of the naturally occurring elements. Normally cesium is stable, but radioactive isotopes of cesium are created during nuclear fission of uranium in either nuclear power plants or nuclear weapons. There has been talk recently that radioactive materials, like cesium 137, could be used in a "dirty" bomb which is a conventional bomb that is designed to spread radioactive materials upon exploding. Although making a dirty bomb is relatively straightforward, making one that would cause a lot of harm is more difficult so the main use of a crude dirty bomb would be to scare people more than a regular bomb would.

I'm sure the military and others have done experiments with cesium to see what it does and how it can be used but I'm not familiar with anything specific that the military has done with cesium.