The invasion of afghanistan by american troops

Caused the Taliban's government to fall quickly.

Explanation:

The Taliban and al-Qaeda had been considered terrorist organizations since the 1990s. However, an invasion was not considered until the September 11 attacks. Afterwards, the United States and its allies launched Operation Enduring Freedom against the Taliban in October, 2001. This caused the Taliban to fall in December, 2001. However, the war was far from over, as retreating proved difficult and al-Qaeda remained strong and involved in fighting.