Attacking a friend or a neutral nation
Before the shooting starts, their leader calls and asks what on earth you are doing.
Right click a friendly or neutral group and choose Attack. When your forces reach them, the game pauses and their leader calls you, demanding an explanation.
Ignore the call or refuse to explain, and it is war. A good explanation or an apology may settle it, though they might ask for money to make up for the trouble.
Excuses are checked against what really happened, so lies can be seen through. Attacking an ally ends your alliance, whatever they say.