First, lets look at the map:
Does anyone notice any nations that Iraq borders? Iran, Saudi Arabia and Syria come to mind.
Does anyone notice which nation is sandwiched between Afghanistan and Iraq?
Imagine trying to win the War on Terror (aka the War on Jihadism) with Sadaam Hussein still in power. Imagine trying to win the War on Terror with Sadaam Hussein still paying Palestinian suicide bombers to keep the region destabilized.
It simply wasn't possible.
Now imagine a successful democracy next door to Iran, Syria, and Saudi Arabia.
Iran is poised for another revolution, and if they see a successful secular Shiite democracy next door, that revolution is on its way.
Saudi Arabia is on a collison course with a violent revolution. 20 years ago, their per capita income was over $28,000. Today its under $7,000. This is one mismanaged nation with no intentions to reform. There is one obvious alternative to the Saudi Royal family: a Wahabi theocracy. There needs to be another alternative, and it needs to be broadcast into the homes of the Saudi people. If Al Jazeera sees a successful democracy in Iraq, that is independent, they will report it.
And then there is Syria. They border two other nations that should be mentioned: Israel and Lebanon. They've been an occupying and destabilizing force in Lebanon for many moons. They have been a perenial state sponsor of radical anti-Israeli terrorists for even more moons.
Look at the map again. Look at Iran, Iraq, Saudi Arabia and Syria as one block. Imagine these nations no longer depriving their people of basic human rights, and then deflecting governmental incompetence by blaming the U.S. Imagine the Israeli-Palestinian conflict resolved, which would resolve the primary grievance that Arabs have with the U.S.
Now imagine these societies reintegrating into the world economy, after several centuries out of it.
What you end up with is an economically interdependent region that is once again prosperous and peaceful. The attraction to Jihadism will evaporate, and with it the terrorist threat to America.
This is why invading Iraq had everything to do with 9-11. If the underlying reasons that allowed Al Qaeda to execute 9-11 disapear, future 9-11s will simply not happen.
Its geopolitcal, and its that clear.