BASRA, Iraq, Jan 19 (Reuters) - U.S. Defense Secretary Robert Gates arrived in Basra on Friday, making an unannounced trip to meet U.S. and British military commanders and other officials in southern Iraq. A Reuters reporter travelling with Gates said he was expected to meet U.S. commander General George Casey among others. It is Gates's second visit to Iraq since being appointed to succeed Donald Rumsfeld, who resigned in November.