World leaders from over 50 countries descended on Washington this week for President Obama’s final Nuclear Security Summit.

Through Obama’s two terms in office he’s used the Nuclear Security Summit to advance the U.S.’s goals of curbing nuclear proliferation. Only nine countries are known to have nuclear weapons, including the five members of the National Security Council. The country believed to have the greatest nuclear arsenal, Russia, is notably absent from this year’s summit, however.

The nuclear deal the U.S. and other permanent members of the United Nations Security Council ironed out is a nod to the efforts promoted at the summit, but with the threat of terror groups like ISIS getting their hands on nuclear weapons looming large, this year’s event has taken on new focus. In the wake of the attack in Brussels, the interest in preventing ISIS from gaining more power has only grown.