easy question - help with attacking

[abkaiser]

I have (what should be a very basic) question about attacking other ships:

Why can't I do it?

I have a AI opponent that has a bunch of ships. I use the "Attack" button to send my ships there, thinking we'll immediately go into battle. No luck.
Okay, then I click "Next turn", thinking that all battles will kick off if ships share the same sector at the end of my turn. No luck here either.

Then... I realize that I have a treaty with the guy I'm trying to attack. But I realize that this AI has attacked me in the past when we were at a Trade treaty with each other.

What's going on? I wanna attack! Why can't I?

Thanks,

Andy