I was able to win the mission like this:
1) Create 1 or 2 big fleet(s) with all the ships and move it/them next to Proxyma ursae.
2) Don't colonize mars
2) Create a military starbase in order to have Haven in range.
3) Try to survey all the anomalies and collonize all the uncolonized planets
4) Put your fleet next to Haven to destroy new enemy ships then invade Haven when you have enough firepower.
5) Invade the xendar's homeworld

NB: Proxyma ursae must be an industrial world in order to create most of you ships.

Is colonizing Mars some kind of AI behavioral trigger?

oh, well yes, naturally mars is one of the last planets i would colonize in any game, but colonize it i will..... just thought if there was some kind of AI trigger associated with it, then no problem leaving it be until i am good and ready for whatever challenge colonizing it created!