Every mission objectives change. In halo the objectives are always the same attack enemy base unless, you are palying the defensive team in invasion. In star wars republic commando it is different per mission. In call of duty it is always attack unless, the commanding officer  ( highest ranking united player ) says otherwise.

Star Wars Republic Coammando Objectives

Hangar: Defend the vents.

Gunship: Attack enemy base.

Ghost Ship: Attack enemy base.

Depot: Attack enemy territory.

Engine: Atatck enemy position.

Garden: Attack enemy base.

Arena G9: Attack enemy base.

Arena A17: Attack enemy base.