Cutting off aid to Afghanistan didn't win us any friends, but OBL said it himself, the 9/11 attacks were about America's occupation of Muslim holy lands and support of Israel.
At the time, Afghanistan had no central government to aid. Hekmatyar, Dostum, and Massoud were turning Afghanistan into a buzzard's feast of chaos after they'd defeated the Najibullah regime. That's why the Taliban got so much local support when they restored law and order. Of course, the Pakis were happy to send arms, cadres, and money north to establish a tame Muslim client state to their north.
Preventing 9/11 would require several PODs IMNSHO:
- A much more cooperative culture between the CIA and FBI.
The FBI was a little slow on the draw, but if they the CIA info in their hands about Al-Qaeda and so forth, they might've moved a lot faster. I'm of the opinion we could've wrapped up the plotters before they even took off.
- The USA making it clearer to Saddam not to invade Kuwait. No Desert Shield/Storm, no stationing troops in Saudi.
- Interrupting the Saudi $$$ pipeline that finances so much of the terrorism of India and "dissident" Muslims not into the Wahhabi way. If the Saudis paid a real price- say, an oil embargo and listing as a terrorist state as they richly deserve, it'd stop. It's the classic false-front strategy. A Muslim charity is set up to help the suffering Afghans, Gaza Strip Palestinians, etc. 10-20% is real medical, education, infrastructure help. The bulk of it is supporting terror groups to intimidate Western countries and Muslim opponents. The brutal irony of it is that WE set it up to give ourselves deniability supporting the mujahedeen.