And yet how many terrorist attacks were there on American targets between Afghanistan and the actual invasion of Iraq? Claiming this would happen in the event of a failure to invade Iraq strikes me as pretty specious reasoning since Saddam's regime didn't have nearly as much to do with backing radical Islamic terrorism as say Iran, Syria, or Sudan have. At most Saddam sent money to families of Palestinian suicide bombers, and backed the Iranian terrorist group MEK--which opposed the regime in Iran. As for the matter of Al-Qaeda troops rushing over to Iraq, according to the this biographical sketch on Al Zarqawi they were largely entering via the Kurdish north where Al Ansar Islam was set up. This is an area by-the-way, which was outside of Saddam's sphere of influence thanks to the no-fly zones. (I'll quote the relevant portion to save paragraph hunting, though if you want to it's about thirteen paragraphs down):Iraq, on the other hand, was ideally situated. In addition, in the wake of the U.S. invasion of Afghanistan, the Al Queda troops were rushing over to Iraq.
If Jamestown's article is be believed, the real influx fighters didn't come until months after the invasion. I also read a Times online report which states there were several Sunni organisations in the Kurdish north, all of which were opposed to Saddam. Though in support of your arguments it states that according to Dr Muhammad al-Masari, a specialist in early Al-Qaeda ideology, Saddam began contact with such groups in early 2002 fearing an American invasion. However, it seems that his claims are disputed and Iraq was only have found to supported Al-Ansar insofar that it used sleeper agents to direct it towards undermining the PUK. Yet there was never any actual collaboration. Moreover, it doesn't jibe with the conclusions reached that there was no evidence that the regime actually aided Al-Zarqawi.Weeks later, Zarqawi was obliged to relocate to a remote area of northern Iraq controlled by the Kurdish Islamist group Ansar al-Islam. A number of Arab Islamists had already set up camp in the mountainous enclave and Zarqawi quickly joined forces with them. Believing that an American invasion to oust Saddam Hussein was inevitable, Zarqawi began preparing the groundwork for the battle ahead. He spent a considerable part of the summer in Baghdad and the so-called Sunni triangle of Iraq, apparently to establish local support networks. Since Iran was no longer a reliable conduit for the travel of Tawhid operatives, Zarqawi spent time in Syria setting up an alternate route.
During or shortly before the American-led invasion of Iraq in March 2003, Zarqawi returned to Iran, where he met with bin Laden's military chief, Muhammad Ibrahim Makawi (Saif al-Adel), who asked him to coordinate the entry of al-Qaeda operatives into Iraq through Syria. Zarqawi readily agreed and by the fall of 2003 a steady flow of Arab Islamists were infiltrating Iraq via Syria.
On the other hand studies by both the U.S. military and Israeli intelligence found that of the captured would-be foreign fighters found crossing the border from Saudi Arabia and into Iraq, who had been recruited by Al-Qaeda, only a small minority had taken place in previous insurgencies. In fact the bulk were radicalized by the Iraq war itself. It seems questionable to assert that what was driving the terrorists on was that the US would appear weak if it failed to invade Iraq, when the invasion itself has simply given them more recruits. In fact this reminds me of how well supporting the Shah worked. It merely gave Ayatollah Khoemini a ready base of support.