I read this short story 30 years or more ago.
A space ship is returning home to Earth from a battle. There are no people on board this ship, only an AI and the ship's computer. When the ship tries to contact Earth it gets no immediate response. The ship attempts to make contact for some time and then is successful with Houston Control (I think).
The AI discovers that it is on a different Earth from the one it is from and that it has also traveled back in time. (The enemy the fleet that it fought still exists).
The space ship comes from an Earth were the Greek empire did not get destroyed.
In the end the AI crashes the ship on the Capitol and gets destroyed, but the computer survives so that the human race has the knowledge of how to build ships and defend against the enemy.
Does anyone know the name of this story?