As Hillary Clinton made a historic visit to Yemen, Vice President Joe Biden made an unannounced trip to Iraq early Thursday morning to discuss the future of U.S. troops in the country. His visit marked the first by a top U.S. official since Iraq approved its new cabinet last month. Under a security agreement between Washington and Baghdad, American troops are set to leave Iraq by the end of 2011, yet Iraq's top military commander says U.S. troops should stay on until Iraq's security forces can defend its borders—though he admitted this might not be possible until 2020. Meanwhile, Iraqi Prime Minister Nouri al-Maliki is still vouching for U.S. troops to leave on schedule. U.S. military leaders have said privately that they will begin to bring some of the 50,000 total troops home in early spring.