After the closing of the Unicron Singularity and Galvatron's death in a duel with Optimus Prime on the Moon, the few Decepticons who had remained loyal to him up to the end joined with the Autobots. Some of them later got bored with the Autobots' "peace and love" routine (while another was kidnapped) and left to form the "New Decepticon Army".
They immediately stranded themselves on Mars.