As the Netherlands’ former Prime Minister Mark Rutte becomes the new General Secretary of NATO on Tuesday, his leadership skills will be put to the test quickly as the bloc confronts both external threats and vulnerabilities within the alliance.

Rutte, a figurehead in Dutch politics for several decades and prime minister from 2010 to 2024, is well-known as a consummate diplomat and consensus-builder, able to keep often disparate parties and lawmakers on side.

Ascribed the nickname “Teflon Mark,” Rutte has been adept at avoiding political crises at home during his career and he’s likely to need those skills — and adeptness at managing often tricky geopolitical personalities and relationships — to navigate testing times for the NATO alliance.

Not only is the bloc facing the ongoing challenge of the Russia-Ukraine war — a conflict which shows no signs of ending as Russia commits more troops and spending to the fight — but also heightened tensions with superpower China, and “rogue states” North Korea and Iran.

The Western military bloc also faces uncertainties closer to home, most significantly with the U.S. presidential election next month and the potential for a second term in office for former President Donald Trump, a man who has openly criticized and berated NATO members’ spending and the alliance’s posture when it comes to its main adversary, Russia.

Signalling he would put his powers of persuasion into practice in his new role, Rutte said Tuesday that he would be looking to encourage NATO’s 32 members to increase defense spending — a perennial bugbear between member states despite it having broadly improved among member states in recent years, data shows — and to keep the transatlantic bond in the alliance “rock solid.”

“As Secretary General, I will work on three main priorities,” Rutte said as he took over the reins from Jens Stoltenberg during a handover ceremony in Brussels.

“The first is to keep NATO strong and to ensure our defenses remain effective and credible against all threats. For this we need more forces with better capabilities and faster innovation, this requires more investment, because to do more we must spend more,” he said. “There is no cost-free alternative,” he added.

Rutte said his second priority is to step up support for Ukraine and “to bring it ever-closer to NATO” saying Europe’s security depended on an “strong, independent Ukraine.” His third priority is to strengthen partnerships within the alliance, particularly with the European Union and countries around the world with similar values, Rutte said. “Our security has to be a team effort,” Rutte noted.

“As NATO secretary-general, Rutte will face a different set of challenges” to when he was PM, Armida van Rij, senior research fellow in the Europe Programme at Chatham House said in analysis last week.

“Externally, there is a revisionist Russia and an assertive China. Internally, there are challenges to democracy within the alliance and depleted military supplies. The alliance has just undergone a period of transformation and developed new regional plans through a new force model, which still need to be adequately resourced,” she noted.

Oana Lungescu, distinguished fellow and former NATO spokesperson at the Royal United Services Institute defence and security think tank, commented that, when it comes to defense spending, Rutte will have to “make a strong case not just about figures and percentages, but about the concrete capabilities that are needed to keep NATO nations safe in a dangerous world,” she said in emailed comments Monday.

The possibility of another U.S. administration led by former President and Republican candidate Donald Trump also poses a potential challenge for Rutte, although on Monday he insisted he would be happy to work with either Trump or Vice President Kamala Harris, adding that he had good relations with both.

“I’m not worried. I know both candidates very well,” Rutte told reporters, adding: “I worked for four years with Donald Trump. He was the one pushing us to spend more [on defense], and he achieved because indeed, at the moment, we are now at a much higher spending level than we were when he took office,” Rutte noted.

“Kamala Harris has a fantastic record as vice president. She’s a highly respected leader, so I will be able to work with both,” he said.

Despite Rutte’s conciliatory stance, there’s no doubt that Trump has had a combative relationship with NATO. During his last term in office over 2017-2021, Trump lambasted various member states for not honoring their 2014 commitment to spend 2% of their national gross domestic profit on defense spending.

While campaigning to return to office, Trump rattled NATO members again in February, when he said he would not provide military protection to any member state that had not met its financial obligations to the bloc and would even “encourage” adversaries “to do whatever the hell they want” to that nation.

The comments provoked outrage in the White House, which at the time described them as “appalling and unhinged.” Outgoing NATO Secretary-General Stoltenberg responded that “any suggestion that we are not there to protect and defend all Allies will undermine the security of all of us and put at risk our soldiers, our personnel who are on the front lines to protect the whole Alliance.”

Rutte – described by RUSI analyst Lungescu as “a steady hand to steer NATO through testing times” – is likely to tread carefully with Trump, but a second term of the Republican could be unpredictable for NATO.

“Like Stoltenberg, Rutte is known as a ‘Trump whisperer.’ He is one of the few European politicians who developed a good working relationship with Donald Trump,” RUSI’s Lungescu commented.

“However, a potential second Trump term could prove much more disruptive, with less U.S. aid to Ukraine, more concessions to Russia, and further questioning of the value of NATO.”