Attacking to allies

Hi, I have one question about attacking to allies: is it possible in some way? I want to know this because in the last game I countered a person who allied with me. All went fine until one turn: half of my country was conquered by that player including the capital! And we weren't even at war. And the best: I was completely unable to attack any of his units, only thing I was capable doing was rounding my cities with units, but due 2 turns holding capital rule, I had no possibilities to counter before losing.

Should this be possible?

Allies can take citys with 0 units in then ;^)