I'm playing on 'challenging' difficulty with all AI on 'intelligent' (1.1 beta) and I have watched the following situation unfold -
The Drengi have a massive advantage in military, and are flying ships through my territory to get to an enemy on the other side of my empire. They are winning the space battle easily, but now are just sitting outside the enemy planets killing any ships that appear.
They've gone for about 20 turns without making transports to try and take the completely undefended planets. In the end I got bored, declared war on the losing team and wiped their civilization out completely using only transports.
Has anyone else seen this problem? The only rationale I can think of for this behaviour is that the Drengi transports did not have long enough range to reach the target planets?
And yes, the Drengi do have Planetary Invasion tech
