U.S. troops were on alert at bases across Iraq on Thursday after concluding that a suicide bomber, probably in Iraqi military uniform, was behind the deadliest strike on Americans since they invaded last year. "I am concerned about ... copycat attacks," said Brigadier General Carter Ham, the U.S. commander in Mosul where the bomber killed 18 Americans and three Iraqi National Guards on Tuesday in a pre-Christmas lunchtime blast at a crowded base mess tent. "So we have to be on our best guard over the coming days, weeks and months for that kind of threat," Ham told CNN. He...