I don't think there is one starting point. Think about the reason OBL gave for attacking the World Trade Center - US military bases in Saudi Arabia, in the Islamic Holy Land. And that goes back to the 1st failed attack on the World Trade in 1993 (prior to the Balkans), and was still the #1 stated reason for the successful attack in 2001.
Why were there US military bases in Saudi Arabia? Because of the First Gulf War in 1991. They were used as "staging" and never went away after the War was over, which provided the major impetus for Osama to rally the Wahabi Muslims and call for attacks on the US. It's not a coincidence that almost all of the hijackers were Saudi.
And if you want to go back further you can tie the reasoning for the attack on Iraq in 1991 to the Iran/Iraq war in the 1980's, (when we supplied chemical weapons to Iraq which he used on Iran and on some of his own people), and if you really wanted to root cause that proxy war it goes back to the Islamic Revolution in Iran in 1979 which was a direct result of the CIA overthrowing Mohammad Mosadegh, the democratically elected leader of Iran in 1953 all because he "nationalized" Iranian oil fields that were controlled by the British owned AIOC (Anglo-Iranian Oil Company) aka BP, and if you go back further than that...
Every action has unintended consequences. There isn't one point in time that you can point to and say this started it all.
Blowback is very real, and we never learn our lesson.