Who was the Roman god of war?
In the Roman pantheon, Mars is the primary god of war. He was also considered the god of springtime, agriculture, and social justice. The Romans also worshiped a goddess of war, Bellona, who was considered to have negotiated the relationship between conquest, war, and peace. Though both Mars and Bellona were deities of war, they were not considered a couple. In some legends, they are described as siblings.
People would show respect for Mars by leaving goods at a temple for him or dedicating an animal (such as a bull or pig) to be sacrificed. In these sacrifices, typically only the internal organs were considered to be "for the gods," and eating the meat of the animal was a way for the community to share in a god's blessing. The month of March is actually named for Mars and would have been an important time of year to earn his favor for the coming agricultural season.
Bellona was originally a Sabine (indigenous Western Italian) goddess who was adopted into the Roman pantheon during conquest. She is often depicted wearing a military helmet and wielding a spear. She was considered so important to matters of war and conquest that the Roman senate would hold meetings in her temples, and official declarations of war were made here.
Mars and Bellona have somewhat differing authority when it comes to war. Mars is identified more with the spirit of destruction in war, while Bellona is more to do with valor and triumph.