From Peter Bergen's The Osama bin Laden I Know:
"The Saudi government's decision to allow the introduction of some 500,000 U.S. troops into Saudi Arabia in the summer of 1990 was a defining moment for bin Laden....In the early 1990's he would turn against the royal family, partly because of its corruption, but primarily because of its decision to rely on non-Muslims to defend the holy land of Arabia." (p. 113)
Given Saddam's designs on the Kingdom (see Chapters 6-7), Bush 41 had no alternative but to commit our forces to the defense of Saudi Arabia. But as Bergen points out, the decision of the Saudi's to place the Kingdom's fate in the hands of a Judeo-Christian nation was the match that lit the powder keg that was Osama bin Laden. It remains perhaps the single biggest--but almost completely missed--legacy of Desert Storm.
But what was never inevitable was the ultimate success of the 9/11 plot itself.
One of the most glaring omissions in the media coverage of the 10 year anniversary of the attacks was the almost complete absence of any mention about the intelligence failure that made 9/11 possible in the first place. Not one intelligence community manager who had a role in the failure lost his or her job in the wake of the attacks. Not one. That lack of accountability--the free pass given people like George Tenet and Michael Hayden--is perhaps, next to the shredding of the Constitution via the so-called PATRIOT Act and the FISA Amendments Act, the most pernicious legacy of 9/11.