Who strikes first?

How do you know who is going to move first during games? I will often tell one of my units to attack an enemy's units, but then his other unit will hit my unit before it can move, somehow, yet I attempt to attack one of their units, but it somehow manages to move at the same turn without getting hit like my unit did (even though my unit was moving as well, when it got hit).

There is a mechanic called Turn Block which determines if your units moves or no.

Chances of TB are determined by unit size and Priority. In short, make your most important moves first so they have a higher priority. When I say "priority" I don't mean at which time, but literally the first move you do in the game.

If you want to know more you should search for some guides such as this one: https://atwar-game.com/forum/topic.php?topic_id=24500