BRUSSELS — One of NATO’s longest serving top officials, Secretary-General Jens Stoltenberg, stepped down on Tuesday, handing over the reins to former Dutch Prime Minister Mark Rutte as the military alliance confronts some of the biggest challenges in its history.

Rutte said his priorities would be NATO’s support for Ukraine, increasing defense spending and strengthening partnerships around the globe.

Former Dutch Prime Minister, Mark Rutte, officially took office as head of NATO, replacing Norwegian Jens Stoltenberg, who has held the post for ten years. Mark Rutte was nominated last June by the 32 member countries of the Atlantic Alliance.