Monday’s departure signaled the end of the longest war in U.S. history, a conflict that began shortly after the Sept. 11 attacks in 2001. The Taliban — which controlled most of Afghanistan at the time — refused to hand over Osama bin Laden, prompting the United States, allied countries and local fighters to push the hardline fundamentalist group from power. But the Taliban regrouped and kept carrying out insurgent attacks, and the U.S. military began a two-decade process of countering the Taliban and assisting the new Afghan government, leading to more than 2,000 U.S. troop deaths and over $2 trillion in estimated costs. Amid bipartisan weariness over the war, former President Donald Trump struck a deal with the Taliban last year to pull all U.S. personnel from the country by May 2021, and Biden extended this deadline by several months but still pushed to leave Afghanistan. As the United States began pulling troops this year, the Taliban rapidly took over territory and the U.S.-backed government crumbled more quickly than expected, and the Taliban entered Kabul by mid-August, cementing the group’s return to power in Afghanistan after a 20-year-long effort by the United States to defeat it. The U.S. military spent its final weeks in Afghanistan occupying Kabul’s airport and flying out evacuees, a mission interrupted last week by an Islamic State-linked suicide bombing that killed more than 100 civilians and 13 U.S. personnel, likely the war’s final American troop deaths. Stay Connected Forbes on Facebook: http://fb.com/forbes Forbes Video on Twitter: http://www.twitter.com/forbes Forbes Video on Instagram: http://instagram.com/forbes More From Forbes: http://forbes.com