Briefly, the U.S. enlisted the Saudi's to drive the Russians out of Afghanistan [1980's]. 
Emboldened by their success, the Taliban & al Qaeda then hoped to create a pan-Islamic 
federation. So with the aim of creating hatred for the West and non-Islamic regimes, they 
started bombing U.S. embassies [1990's] - aiming "less at damaging the US than provoking 
it to a reckless assault on Islam", thereby indirectly leading to establishment of the 
foundations of a pan-Islamic federation. The lack of such a U.S. response lead to al 
Qaeda's attack on September 11th, which in turn lead to the U.S. obsession with WMD's 
that might be provided to it's enemies for use on U.S. cities [2000's]. After overtaking 
Afghanistan and dispersing the Taliban & al Qaeda, and knowing that much of the support 
for al Qaeda came from Saudi Arabia, the "invasion of Saudi Arabia presented the tactical 
problem of waging war against a country of vast area and the strategic one of disrupting 
the world's oil supplies. The Americans had established and then strengthened a military 
presence in countries surrounding Saudi Arabia - Yemen, Oman, Qatar, Bahrain and Kuwait. 
Invasion of Iraq would complete the encirclement. From a purely military view, Iraq is 
the most strategic single country in the Middle East, [bordering] six other countries: 
Kuwait, Saudi Arabia, Jordan, Syria, Turkey and Iran."


Source: http://www.theaustralian.news.com.au