KARABILA, Iraq (Reuters) - U.S. forces launched their second major offensive in western Iraq in as many days on Saturday, ratcheting up the hunt for rebels and weapons in the Euphrates river valley that stretches to the Syrian border. Faced with slipping poll numbers, President Bush asked Americans to show patience, saying that since U.S. troops overthrew Saddam Hussein, Iraq had become a "central front in the war on terror" and a "vital test" for U.S. security. Around 1,000 U.S. Marines and soldiers, supported by Iraqi troops and backed by fighter jets and helicopters, launched Operation Dagger in the desert...