A staunch opponent of NATO as a young man, as head of the alliance Jens Stoltenberg has overseen its biggest military expansion since the Cold War and kept sustained pressure on allies to spend more on defence.
The Norwegian, re-appointed as NATO's secretary general until 2020 on Tuesday, has sought to reassure its members alarmed on one side by the newly assertive Russia and on the other by US President Donald Trump's occasional apparent ambivalence about the transatlantic alliance.
Stoltenberg, 58, came to office in 2014, as the Ukraine crisis raged after Moscow's annexation of Crimea, and under his leadership NATO has deployed four combat-ready battle groups -- around 4,000 troops -- to Estonia, Lithuania, Latvia and Poland to counter the Russian threat.
It is a key part of the twin-track "deterrence and dialogue" strategy towards Moscow that Stoltenberg has promoted.
Former Norwegian prime minister Stoltenberg -- the first NATO secretary general from a country bordering Russia -- often refers to his track record of negotiating with Moscow, stressing the importance of maintaining contact, but doing so from a position of strength.
And he has consistently pressed NATO members to hit a target, agreed in 2014, of spending two percent of GDP on defence -- an issue that has rankled with Washington particularly since Trump came to power last year.
Before taking over as secretary-general from Denmark's Anders Fogh Rasmussen, former Norwegian Labour Party chief Stoltenberg, an economist by training, had not shown any particular fondness for defence or security matters.
As a young man he was a staunch opponent of both NATO and the European Union, and as a long-haired teenager in the 1970s, he threw stones at the US embassy in Oslo in protest against the Vietnam War.
But under his leadership in the 1980s, Norway's Labour Youth went from advocating the country's exit of NATO to rallying to the cause of the Atlantic alliance.
Born into a political family -- his father was minister of defence and then of foreign affairs, his mother a deputy minister -- the married father of two also devoted the majority of his career to politics.
After entering parliament in 1991, the tall, blue-eyed career politician rose rapidly through the ranks, becoming minister of energy and then of finance, before being named the country's youngest prime minister in 2000, the day after his 41st birthday.
He only kept that position briefly, but returned to power in 2005 and stayed on at the head of government until October 2013.
Under his leadership, the Scandinavian country participated in the war in Afghanistan and contributed to the air strikes against Libya's Moamer Kadhafi.
Oil-rich Norway, traditionally pacifist but with strong Atlantic ties, is one of the few NATO countries that has increased its defence budget in recent years, when most nations were forced to cut down due to the financial crisis.
Enjoying a high level of popularity as prime minister in his own country, Stoltenberg also received international praise when he called for "more democracy" and "more humanity" after extremist Anders Behring Breivik killed 77 people on July 22, 2011.