NATO Secretary General Jens Stoltenberg steps down on Tuesday after 10 testing years for the Western military alliance's unity and purpose.
Stoltenberg will hand over to former Dutch prime minister Mark Rutte with a wreath-laying ceremony and an official handshake at NATO headquarters in Brussels.
Ambassadors from NATO allies will then meet in the alliance's decision-making body, the North Atlantic Council (NAC), to appoint Rutte.
The new Secretary General is expected to start his tenure with an address to NATO staff and a press conference.
The changing of the NATO guard takes place as war rages directly on the alliance's borders in Ukraine more than two years after the full-scale Russian invasion.
The war and increased threat of Russian revanchism renewed NATO's Cold War purpose to protect members after years of turmoil during former US president Donald Trump's presidency.
Trump railed against allies, especially Germany, for what he deemed as unacceptably low defence spending and questioned the alliance's principle of collective defence.
French President Emmanuel Macron also infamously declared that the alliance was in danger of succumbing to "brain death," such was the degree of division among NATO.
A chaotic withdrawal from Afghanistan in 2021 marked a low point for the alliance, just six months before Russian President Vladimir Putin's fateful invasion in February 2022.
In an essay to mark his departure, Stoltenberg described the last decade for NATO "as the most transformational" for the 32-member alliance since the Cold War.
Stoltenberg highlighted the new deployments of allied troops in Eastern Europe, the ramp up in defence industry production, and that 23 allies are now spending at least 2% of their gross domestic product (GDP) on defence in 2024 - up from just three countries in 2014.
The NATO secretary general was credited with moderating the debate over low defence spending by the European allies, which led to heated disputes during Trump's time in office.
During Stoltenberg's tenure NATO also welcomed four new allies, Montengro, North Macedonia, Finland and Sweden. Both Nordic countries abandoned years of neutrality after the all-out Russian invasion of Ukraine.
Stoltenberg's skillful shepherding of Finland and Sweden into the alliance in the face of Turkish resistance was one of his crowning achievements.
A staunch supporter of Ukraine, allies extended Stoltenberg's term in office just after the 2022 Russian invasion. NATO extended his tenure in 2023 again as allies were unable to agree on a suitable successor.
Rutte quickly became the frontrunner to be the next NATO chief after receiving the early support of the United States, Germany and Britain.
This was despite criticism of low Dutch defence spending during Rutte's time in office.
The centre-right politician was the longest-serving prime minister in Dutch history. His initial term as NATO Secretary General is to be four years long.
The top NATO official is tasked with the political leadership of the alliance and ensuring consensus within the alliance, particularly when navigating complex issues during times of crisis.
In addition to support for Ukraine, the potential return of Trump to the White House in November would be a major test for Rutte. Past statements from Trump have cast doubt on US collective defence commitments to the alliance.
NATO is also concerned about the rising military strength of Beijing and views China as a challenge to the alliance's security.
