The US says Iraqi troops are posing as civilians
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The apparent suicide attack against United States forces near Najaf is not the first time American soldiers have encountered suicide bombings.
But the willingness of a bomber to die can make such assaults particularly hard to guard against.
The first suicide bomb aimed at US soldiers was also the biggest.
In October 1983 a driver from the Lebanese Hezbollah movement rammed a truck crammed with explosives and gas canisters into a barracks full of US Marines in Beirut.
The assault left 241 Americans dead, hastening the end of what was supposed to be a peacekeeping mission.
A similar attack on the same day killed 58 French soldiers.
Protection difficult
Since then, awareness of the risk from suicide bombers has not always made it possible to protect even well-armed military targets.
A group of al-Qaeda attackers in a small boat proved that by hitting the US warship Cole in Yemen three years ago, blasting a hole in its side and killing 17 sailors.
And the attacks - again using truck bombs - on the American embassies in Kenya and Tanzania showed that some suicide bombers are willing to kill many civilians, unconnected with their target, in order to strike at US interests.
More than 200 people died, of whom fewer than 20 were Americans.