President Joe Biden announced the United States killed al Qaeda leader Ayman al-Zawahiri in a drone strike on Saturday.

“He was deeply involved in the planning of 9/11, one of the most responsible for the attacks that murdered 2,977 people on American soil. For decades, he was the mastermind of attacks against Americans,” Biden said Monday in a speech from the White House.

“[Ayman al-Zawahiri] will never again allow Afghanistan to become a terrorist safe haven…It can’t be a launching pad against the United States,” Biden continued.

Biden defended his decision to kill the al Qaeda leader because, “As commander in chief, it is my solemn responsibility to make American safe in a dangerous world.”

Watch Biden’s complete speech below.