The Fetials were a small collection of priests who's responsibility it was to conduct the rites involved with declaring war on another nation. They seemed to have functioned with importance in the early years of Rome, but gradually lost their application as the Republic fell and Empire remained. There was an area of land of one the temples...Bellona? that was considered to be foreign territory so that these priests could conduct the ritual when Rome declared war on a distant enemy.

I've posted some Livy describing the ritual.