In 2001, the September 11 attacks occurred. Which war began soon after?

The story behind the answer

The war in Afghanistan began just weeks after the September 11, 2001 attacks. On October 7, 2001, the United States, with allies, launched Operation Enduring Freedom to target al‑Qaeda and remove the Taliban regime that sheltered it. This makes Afghanistan the immediate post‑9/11 war. In contrast, the Iraq War started later in 2003; the Gulf War occurred in 1990–91; and the Korean War in 1950–53.

Context to remember: NATO invoked Article 5 (collective