They didn't. Ares came first
Mars is the Roman name for Ares.
mars
Ares or Mars the Roman name for Ares symbol is a spear for war.
He was called by the Greeks "Ares" well before the Romans called their Mars the same as Ares.
The greek name for Mars would be Ares, the god of war and the roman name would again be Mars.
Ares
mars
Mars
Mars (the Roman God of War) was based on Ares (the Greek God of War).
Mars=Ares- god of war
Mars's Greek name was Ares.
The name Mars comes from the Roman god Mars which in Greek is Ares.