Convert every system to the Republic. (Single Player Only)

To access Ground Battle for the Republic, you must have access to particular control console. To get there from the main hub, go down the elevator on your left and head to the far right corner with the metal grating. Once through, take the elevator to the right, which requires gold bricks and go through there.

After you get down there, you will need to buy a heavy vehicle to blow through the locked grating in the middle of the room. That is the ground assault console you see through it. Note that all planets in the system must be converted to Republic (Blue) at the same time in order for the achievement to unlock.

While working for the 100% achievement, once every system has been converted to Separatist once, you will unlock Chancellor Palpatine as a playable character. Note that if a system begins under Republic control it must be converted to Separatist and then back to Republic in order to unlock the character.