Early in Rome's history, Rome fought neighbouring cities which had gods similar to the Roman gods. Those foreign gods were often considered to be roughly equal in power to the Roman gods. The Romans undertook elaborate steps to ensure they - and not the enemy - would receive the favour of the gods.
The proces began with the Romans having (or making up) some legitimate cause for war, a grievance of some sort. Before declaring war, they would have a fetial (priest devoted to Jupiter) go to the enemy territory and demand reparations to avoid war. This 'demand for reparations' was essentially blackmail. The demand was non-negotiable and often set at …show more content…
As I said, it is a religious rather than a political proces, and its effect would have been mostly psychological as the Romans would be assured of divine favour. With this in mind, it is easier to understand the subsequent evolution of this proces.
For as Rome expanded and the enemies would be farther away, the three journeys required for one declaration of war became too much. At some point it became accepted that the final journey could be cut from the proces. The fetials would simply declare a small patch of land near Rome to be 'enemy territory' and throw the magical spear there. Later on another journey was cut by making conditional declarations of war. Bring the spear on the first journey and if the response to the demand for reparations is negative toss it in their territory.
By the Middle Republic the process was a skeleton of its former self. Religous backing wasn't even considered necessary if the enemy wasn't considered particulary daunting or wasn't considered worthy. Gallic and Germanic tribes will often have fallen in that category. At other times, all that remained of the process was the conditional part: A legate (a politician) could go to an enemy to bring the conditional declaration of war, and he wouldn't even have to toss a spear to make anything