The planet Mars is named after the Roman god of war. Originally this planet was named Ares, for the Greek god of war, largely because of its blood-red color.
When the Romans conquered Greece, they renamed all the planets for their own equivalent gods, including switching the name of Ares for their own Mars. As Western astronomy evolved, it was natural to retain the Greek and Roman names for the planets.
In 1873, American astronomer Asaph Hall discovered that Mars had two small satellites. He named them Phobos, or fear, and Deimos, or terror. These names were from the twin sons of the god Mars who drove his chariot into battle and who were his constant companions.