BRUSSELS—Five months ago, Jens Stoltenberg was preparing for a cushy job in Norway, running one of the world’s most stable central banks and richest sovereign-wealth funds. Instead, he is now flirting with risks as extreme as nuclear war after signing on for an exceptional fourth stint as NATO secretary-general.
Mr. Stoltenberg, a former Norwegian prime minister, started leading the North Atlantic Treaty Organization in 2014, months after Russia seized the Crimean Peninsula from Ukraine and fomented a rebellion in the country’s east. Since then, he has overseen the biggest transformation and expansion of the Western military alliance since the Cold War.