Afghanistan is a landlocked country situated between the Central Asian republics (Turkmenistan, Tajikistan, Uzbekistan), Iran, and Pakistan. There is also a teeny-tiny land border with China. 

Historically, Afghanistan has been invaded many, many times... but almost never successfully. Alexander the Great crossed Afghanistan on his way to conquer the Indian subcontinent, but he was struck by an arrow from an Afghan tribesman and died of his injury before he could achieve his goal. The British invaded Afghanistan multiple times, with little success, as did the Soviet Union in the late 1970s. During the Soviet/Russian invasion, the United States supplied resistance groups with military training and weapons... some of which would be used against the United States when the U.S. invaded in 2001. 

After the U.S. was attacked by terrorists on September 11th, 2001 (Tr. Larry's first day of high school, btw) a group called Al-Qaeda took responsibility for the attacks. Al-Qaeda was led by a man named Osama bin Laden, and he was known to be hiding in Afghanistan. The ruling Taliban government had allowed bin Laden and his group to exist within the country.

The U.S. invasion was initially to destroy terrorist training camps, hopefully find Osama bin Laden, and to damage the Taliban government enough to ensure that they would not be able to support other terrorist groups. The U.S. declared an end to "major combat operations" in 2003, but stayed in the country until the end of August 2021 to support the Afghan government, train the Afghan military and security forces, and aid in the development (and re-development) of the country. 

The Taliban had lost power in 2001, but were never totally "defeated." They always retained some control over far-off regions of the country, and in the mid-2010s began to retake territory and grow influential. In 2020, the U.S. President Donald Trump began negotiations between the U.S. and the Taliban, but left the official Afghan government out of those negotiations. Part of the agreement required the U.S. to remove its military from the country; President Joe Biden continued with this plan when he became President in January 2021.