Iraq is also the operating base of an insurgency of up to 20,000 fighters, comprised of Saddam loyalists and other foreign Islamic fighters, including al-Qaeda terrorists. These groups are receiving support from Iran, Syria and others who don't want to see freedom and democracy flourish in Iraq.
Are Iran and Syria sneaking soldiers and supplies into Iraq? Well, let's consider the border with this map collected and put online by the University of Texas. The border of Iraq and Iran appears to offer some hope of getting supplies secretly through as there are some fair-sized heights. The map lists Gundah Zhur on the Iraqi side as being the highest elevation in the area at a little over 3600 meters. But by and large, the mountains appear to stop at or near the Iraq border, with the most mountainous areas being well inside Iran. The border with Syra appears to be more or less flat, with no significant heights at all. The Land Use map shows that there are indeed some forested areas up North, but that "Meadows and pastures" stand between them and the border. There are a few spots where the forests come right to the Iran border, but the North is largely Kurdish anyway and they're the most pro-American group in Iraq. The fact that all parts of Iraq lack the triple-canopy rain forest of Vietnam/Laos/Cambodia means that very little smuggling is likely to be taking place.
A Times of London article states that the Syria/Iraq border is the site of frequent trade, so there might be small arms and some people coming through, but the numbers are probably not large. Unlike in Vietnam, we are not likely to see whole brigades and regiments crossing into Iraq. This general impression appears to be confirmed by an Atlanta Journal-Constitution article:
Bureaucrats and politicians in Washington try repeatedly to pretend otherwise, suggesting that al-Qaida-linked terrorists are pouring into Iraq from Syria, Iran and even Saudi Arabia to attack our troops. But U.S. generals in Iraq, the people actually doing the fighting, have said repeatedly that they have seen little evidence of international involvement. Furthermore, the captains, majors and colonels charged with guarding Iraq's borders report no influx of foreign terrorists into Iraq and are puzzled by claims to the contrary.
So it appears unlikely that there's much coming in. Unfortunately for the Bush Administration, the only other explanation for the guerrilla conflict is that the occupation policy has sparked a lot of the war. Failure to secure government offices and hospitals from looters left what remained of the Iraqi government unable to handle normal post-war tasks. Failure to secure museums and other cultural treasures was a propaganda windfall for guerrillas who were waiting to see how the occupation would unfold. The fact that President Bush asked for $18 billion for reconstruction, received it and then let it sit idle (To this day, only about 5 – 10% has been spent) has allowed garbage to pile up in the cities, people to sit around with nothing to do, tasks to go undone and power plants to remain performing considerably under-capacity, leaving looters and criminals free to roam about. US arrest procedures were humiliating from the start and the atrocities of Abu Ghraib have infuriated Iraqis even more, greatly adding to the ranks of the resistance. The Observer from Britain adds detail to the picture of how ordinary Iraqi citizens became insurgents. The attack on Fallujah in April was seen as a grave error by the Marine general in charge. US forces, reacting to the grim atrocity of resistance fighters killing and then citizens tearing apart four contractors, or mercernaries. We don’t know who ordered the attack on the city of Fallujah, but it seems certain to have come from the White House.