What is the name for the college of 20 priests responsible for formal declarations of war?

The correct answer is the Fetiales, who were a college of priests in ancient Rome assigned with the responsibility for the formal declaration of war and conducting peace treaties. They played a crucial role in ensuring that the rites and rituals governing warfare were conducted properly, which included the sacred requirement of consulting the gods before declaring war.

The Fetiales would carry out rituals that included the act of throwing a spear into the enemy's territory, signifying the formal initiation of hostilities. This was more than just a ceremonial act; it reflected the belief that warfare was a matter of divine order and required religious justification.

Other groups mentioned do not fulfill this specific role. The Augures were priests who interpreted the will of the gods by studying the flight patterns of birds. The Quindecemviri were tasked with maintaining the Sibylline Books, which were prophetic texts, and did not have a direct role in war declarations. The Salii were priestly officials associated with ceremonial dance and the rites of Mars, the god of war, but they did not directly declare war.

Thus, the Fetiales are specifically recognized for their duty in formalizing war declarations through sacred rituals, making them the accurate answer in this context.