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

Stoltenberg, NATO’s 13th secretary-general, took over in 2014, the year that “little green men” from Russia infiltrated Ukraine. Moscow annexed the Crimean Peninsula, sparking a defense spending buildup at the world’s biggest security alliance that gathered pace over his term.

His tenure was surpassed only by Dutch diplomat Joseph Luns, who spent 12 years in charge of NATO.

A series of events to mark the handover will be held at NATO headquarters in Brussels, Belgium. NATO secretaries-general run the HQ, drive the alliance's working agenda, and speak on behalf of the 32-nation organization with one unifying voice. Continuity is usually the key word when they take up office.

Rutte becomes NATO’s top civilian official as Russia’s full-scale invasion of Ukraine approaches the 1,000-day mark. Political uncertainty within the organization is also high, as election fever roils the United States, its unofficial “chairman of the board” and most powerful member.

Russian forces are making advances in eastern Ukraine. Ukraine’s army has a shaky hold on part of the Kursk region in Russia, which has provided a temporary morale boost, but as casualties mount it remains outmanned and outgunned.

Rutte will have to find new ways to encourage support for Ukraine among the allies, whose ranks have swelled to 32 countries since the invasion as Finland and Sweden joined to seek protection from Russia under NATO's security umbrella.

Across the Atlantic, surveys suggest the U.S. election in November will be a close race. It could see a return to the White House of Donald Trump, whose bluster during his last term of office about low defense spending among European allies and Canada undermined trust of NATO member countries.

It became an existential challenge, as smaller members feared that the U.S. under Trump would renege on NATO’s security pledge that all countries must to come to the rescue of any ally in trouble, the foundation stone the alliance is built on.

Trump has been critical of U.S. aid to Ukraine, and he suggested last week that President Volodymyr Zelenskyy should have made concessions to Russian leader Vladimir Putin before he launched his invasion in 2022.