WASHINGTON — It is unclear who will take over at the Pentagon and the military services when the top leaders all step down Monday as President-elect Donald Trump is sworn into office. As of Friday, officials said they had not yet heard who will become the acting defense secretary. Officials said the military chiefs of the Army, Navy and Air Force were getting ready to step in as acting service secretaries — a rare move — because no civilians had been named or, in some cases, had turned down the opportunity. [time-brightcove not-tgx=”true”] As is customary, all current political appointees will step down as of noon EST on Inauguration Day, leaving hundreds of key defense posts open, including dozens that require Senate confirmation.