"These Shiite militias have emerged as perhaps the greatest threat to the 46,000 United States troops still in Iraq, military officials say. And a barrage of recent attacks — some of them deadly — has raised questions about the safety of Americans as the military withdraws troops and equipment in the months ahead. “There are plenty of groups who will be paid to kill the last Americans on their way out,” said Col. Douglas Crissman, the military commander who oversees Maysan and three other southern provinces. Officials say the attacks, coupled with an increase in anti-American leafleting and speeches by hard-line groups, seem to be aimed at tilting the highly charged public debate over whether American forces should be asked to remain in Iraq despite a deadline to leave by the end of the year. Mr. Sadr himself makes no secret of his strategy. “Yes, we are still resisting and striking bases, troops and vehicles, as long as they are in Iraq,” he told the BBC Arabic service on Thursday. “And there is no doubt with that. It’s an honor for us.” Southern Iraq is strategically important to the United States, even in the final days of the American deployment here. It is the point of entry for many of the weapons coming from Iran, particularly rockets and the shaped explosives used in improvised explosive devices, or I.E.D.’s, military commanders say, and thousands of departing troops and convoys will pass through the region as they head into Kuwait. Last week, militants hit a United States military base in Basra from seven miles away, and in a single day about 10 rockets were fired at the Green Zone in Baghdad, home to the American Embassy and a sprawling American military base. American officials say many of the militants have close ties to Iran or to Mr. Sadr, whose once-fearsome Shiite militia, the Mahdi Army, was largely demobilized after suffering humiliating defeats three years ago."