1. The Invasion of Afghanistan in 2001, shortly after the attack. It lead to the Taliban government in Afghanistan to be overthrown.
2. The invasion of Iraq. While it happened in 2003, a couple years after the initial attack, the attack prompted the U.S. to be more suspicious of Saddam Hussein, then leader of Iraq. The U.S. invaded, deposed Saddam, and then a massive power vacuum lead to the rise of ISIS as we know it, and other factions.
3. The Department of Homeland Security was formed because of the attack.
4. Years upon years of fighting in the Middle East occurred, as the U.S. sent it’s military forces into the area after the attack.