Iraqi leaders are becoming more aware of their vulnerability to Iranian (or Turkish or Saudi Arabian) invasion, as American forces prepare to completely withdraw. There are two problems here. One is that the Iraqi Army has, for the last five years, been used primarily as an internal security force. With 230,000 troops and 18 divisions, the army is short of armored vehicles and artillery. More importantly, the army has not trained to oppose an invasion from, say, Iran.
The Iranian Army is twice the size of Iraq's, with twice as many divisions, a lot more tanks and artillery, and training for offensive operations. But the Iranian armed forces suffer from decades of arms embargos, and spend a lot of time keeping elderly equipment and weapons operational. Moreover, the kinds of troops movements needed for an invasion of Iraq would be detected by American intelligence (the spy satellites alone would catch this.) American air power could be beefed up in Kuwait and Saudi Arabia, and the other Arab states in the region would likely offer help (in the form of air power) to Iraq. Iran could still invade, and might be able to grab many Iraqi oil fields in the south (which are close to the border). This is highly unlikely, as the counterattack (from the air) would devastate the Iranian economy. Being the aggressor would lose Iran any popular support around the world, or even inside Iran.
Turkey has a much more modern force, but invading Iraq would still bring economic and military retribution. The Turks are not known for doing really stupid things, like suddenly deciding to invade a neighbor. The Saudis also have a lot of reasons not to invade, and very few incentives to move north.
Thus the Iraqis, while nervous about their military vulnerability once the Americans leave, don't believe any of their neighbors are crazy enough to invade. And even if an invasion occurred, for once Iraqi would be on the side of the angels, and eligible for rescue by the United States and the world community. Then again, there's also a nightmare scenario, where the Iranians and Turks partition Iraq with a joint invasion (Iran gets southern Iraq and Baghdad, Syria gets Western Iraq and Turkey gets the Kurdish north and the oil wells up there.) Iran has recently revived its centuries old claim on southern Iraq, and Turkey still resents the loss of northern Iraq (and its oil) as punishment for losing in World War I. These old grudges often create unpleasant decisions.