You mean the Red Tide/Blitzkrieg Mode?
Don't change the game settings. If you want to play it, it should be on the Victory Point setting.
I played a game like that, the objective was to "Defend the base" but with no timer or no in-game prompt...
So after spamming tank hunters I pushed the enemy panthers back, then started making lots of tanks and destroyed the enemy...completely. Their base, their MGs, I had every point secured, but the game wouldn't end. I had to quit