Defense Secretary Donald H. Rumsfeld has returned to Washington after his surprise trip to Iraq. Rumsfeld's press secretary, Eric Ruff, said the defense secretary did not meet with any Iraqi government officials - as has been his usual practice - before leaving the country around noon Sunday. "It was not a public trip whatsoever," Ruff said. The main purpose, he said, was to express thanks to the US troops and their commanders. Rumsfeld remains defense secretary until Dec. 18.