I brought two of the tanks safely to my base but then nothing happens, I can't complete the mission... I even tried destroying the enemy to see if that did anything.

What can I do? I just want to move forward.

I've heard about this quite a lot... a simple google search will solve your problem, probably. You could also try http://www.tauniverse.com/forum

Sorry for not being specific, but my point is that this is quite common (probably).