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STORY: Mark Rutte, NATO's next secretary general, is a fierce critic of Russian President Vladimir Putin and a staunch ally of Ukraine.

Rutte will face the challenge of sustaining allies' support for Ukraine's fight against Russia's invasion but must guard against NATO being drawn into a war with Moscow.

So how will he run the alliance?

:: Ally of Ukraine

Rutte has spent nearly 14 years as Dutch prime minister and cemented his bid to become NATO's new chief last year while co-leading an international coalition that will deliver F-16 fighter jets to Ukraine and train Ukrainian pilots.

He has been a vocal critic of Putin since Russia's 2022 invasion of Ukraine.

"Speak out against Putin and Russia's violation of the U.N. Charter. Support a peace plan, support Ukraine, even if it takes time, even if there are setbacks, especially if it takes time and there are setbacks, and tell Russia to give back the stolen children of Ukraine."

He says NATO must be powerful to counter Moscow, and other European Union leaders must not be naive about Putin's Russia.

:: Transatlantic challenge

Rutte is due to take over as leader when incumbent Jens Stoltenberg officially steps down in October.

Almost immediately, he will have to contend with the possibility that NATO-skeptic Donald Trump may return to the White House after November's U.S. presidential election.

The prospect of Trump's return has unnerved NATO leaders as the Republican former president called into question U.S. willingness to support other members of the alliance if they were attacked.

This was Stoltenberg in June.

"Article Five (of the North Atlantic Treaty, NATO's collective defense clause) is a legal obligation for allies to assist in the event of an armed attack. There is no requirement for any unanimous decision."

It led Stoltenberg to propose what is widely seen as "Trump-proofing," by putting coordination of international military aid for Ukraine under a NATO umbrella, giving alliance military staff more autonomy to get on with the job with less direct political involvement.

As prime minister of the Netherlands, Rutte is regarded as having been one of the most successful in the EU at dealing with Trump, in spite of the latter's sympathy with Russia.

Rutte will oversee a widened NATO, after Finland then Sweden turned their backs on long histories of non-alignment following Russia's invasion of Ukraine.