Which planet is named after the Roman god of war?

Mars, for the colour - the rust-red iron oxide covering its surface suggested blood to ancient observers, and the association with war is much older than the Roman name. Babylonian astronomers called it Nergal, after their god of war and pestilence.

Its two moons keep the theme and are stranger than the planet. Phobos and Deimos - fear and dread, the sons of Ares - are small, irregular and probably captured asteroids, or possibly debris from an impact. Phobos orbits so close and so fast that it rises in the west and sets in the east twice a day, and it is spiralling inward: within perhaps fifty million years it will either strike the surface or be pulled apart into a ring.

Jonathan Swift described Mars as having two moons in Gulliver's Travels in 1726, 151 years before they were discovered, and got the orbital ratios roughly right. This is regularly presented as uncanny and is not: Kepler had speculated about two Martian moons a century earlier on a numerological argument that Earth had one and Jupiter four, and Swift was satirising exactly that kind of reasoning.