The bombing of an American mess hall on the 21st was a suicide bombing operation. This attack killed 22 people, including 13 U.S. troops. This is the first time such an attack has been carried out on an American base. That's because the security for bases has been very good. The suicide bomber was wearing an Iraqi army uniform. The Baath Party secret policemen have long experience in getting Iraqis to do their bidding, and it's surprising it took this long for them to get a suicide bomber into a base. The basic technique for recruiting a suicide bomber is to find someone who is already in the Iraqi army or police and has access to a base. Then you kidnap a close relative and tell the guy to either carry out the mission, or the kinfolk die. Not all people are willing to die for a relative, so in some cases, the plotters will deceive the bomber. This means making the bomber think he is just delivering the bomb, not wearing it when it goes off. This kind of viciousness is rare in human history, but common in the Middle East, and Middle Eastern history.
The mess hall was a tent like facility, and it was first thought that a rocket or mortar did the damage. But close examination of the debris quickly revealed what actually happened. The Mosul camp was due to have the mess tent replaced by a bunker, which would have provided better protection from the usual rocket and mortar attacks. Actually, a bunker would have led to more casualties if a suicide bomber were involved. A tent like structure lets the explosion to disperse, while a bunker contains it. But the major threat are the random rocket and mortar attacks. The most fearsome weapon is the 122mm rocket. This weapon, first developed by the Russians during World War II, is usually fired in bunches from a launcher. Not very accurate, they have a range of over 20 kilometers. But if a dozen or more are fired from a launcher at once, they can hit a large area target. Russia, and its allies, still build these 150 pound rockets. There were thousands of them stored all over Iraq, along with other artillery and rifle ammunition. But anti-government forces can haul one of these nine foot long rockets in the back of a truck, or by two men on foot. A crude launcher can be set up, and sometimes a timer or remote device is used to ignite the rocket. This is not an accurate way to fire the rocket, but if used within a few kilometers of a large American base, it will often land somewhere within the base.