Committing units with a warlord
After rereading the 'Learn to Play rulebook closely, I found the answer on page 11 (emphasis mine):


  When a player wins a battle at any other planet, his warlord (if it
  was committed to that planet) returns to his headquarters, maintaining
  the state (ready or exhausted) it was in at the end of the battle. The
  planet remains in play, and other surviving units remain at that
  planet, also maintaining their state.


In short, any non warlord units at a planet that is not the first, will remain there after the battle is completed.