Why did the United States invade Afghanistan in 2001

because of 9/11 when they had fired the first shot that was our retaliation,

The war in Afghanistan began on October 7, 2001 as part of Operation Enduring Freedom, which was the response of the United States, the United Kingdom, and 45 other states to the terrorist attacks on September 11, 2001. The objective of the invasion was to capture Osama bin Laden and other members of Al Qaeda; the destruction of the Islamist regime of the Taliban, which supported Al-Qaeda and provided it with a secure background; the restoration of peace in the country; the protection of the Afghan population against Taliban threats; and the establishment of a democratic government.

This military action was based on the Bush doctrine, which determined that the United States would not distinguish between terrorist organizations and states or governments that would protect them.