Attack turns

If two players in two cities attack the one another, how it is decided which player attacks and which one cant move his troops and stays at the city to defend?
for example if a player sent some tanks near to my city and i want to attack his tanks while he wants to attack my city, who strikes who? hes tanks on the city or the troops i send from the city on his tanks?

sorry for the confused words, i didnt know how to ask it.;D

By the order in which you move your units, if you move your tanks as your first move, and he moves his tanks on his 4th move, then you will prolly move first, if you both move your tanks first then it is a 50 50 chance who goes first.

It's only a 50:50 chance if there are only 2 players playing.