No one in particular. Russia would be one current example, Israel (and others in the region at various times) another, the US and Germany historically, the Romans, the Ottomans, China, Japan, Britain, Spain, warlords in the western sahara, the kid at school who wanted the other kids' lunch money.

The idea though is that if everyone suddenly disarmed overnight it would be so highly advantageous to a deviant aggressor that one would assuredly emerge.