NEAR AN NASIRIYAH, Iraq - U.S. troops determined to snub out resistance and protect vulnerable supply lines swarmed around this key Euphrates River crossing Saturday, newly wary of the local populace after Iraqis seized a Marine amid heavy fighting to the north. Reports of looting in An Nasiriyah and of the hurried departure of officials of Saddam Hussein's Baath Party, while unconfirmed, gave some hope that Saddam's regime was losing control of the city's 500,000 people. Armed Iraqi militias captured the Marine and destroyed three vehicles Friday night in an ambush of a 200-truck Marine convoy bearing desperately needed supplies...