Brussels — Since assuming the role of NATO secretary general in 2024, Mark Rutte has been tasked with managing a complex and often tense relationship between the alliance and former U.S. President Donald Trump, whose unpredictable approach to international affairs has posed significant challenges to NATO’s cohesion.

In January, tensions peaked when Mr. Trump publicly threatened to seize Greenland from Denmark, a fellow NATO member, and refused to exclude the use of force to acquire the territory. This unprecedented conflict between allies put the alliance’s unity at risk. During a meeting at the World Economic Forum in Davos, Mr. Rutte engaged directly with Mr. Trump, reportedly telling the former president that NATO could not assist with acquiring Greenland but could support security efforts in the Arctic. Presenting existing NATO military exercises in Greenland as part of a broader Arctic mission appeared to temporarily ease Mr. Trump’s stance.

Despite this apparent breakthrough, the episode stirred unease among European members of NATO, particularly Denmark, which viewed discussions about Greenland’s sovereignty as beyond Mr. Rutte’s mandate. NATO officials were reported to have debated giving the United States control over the land beneath American bases in Greenland, an issue the Danes considered a red line. NATO spokeswoman Allison Hart denied that Mr. Rutte discussed or proposed anything related to the sovereignty of any ally.

Mr. Rutte, a former Dutch prime minister with a 13-year tenure and a reputation for pragmatic leadership, is widely seen as a key figure in maintaining engagement with the United States amid Mr. Trump’s often critical posture toward the alliance. The secretary general and Mr. Trump are said to communicate several times a week, sometimes informally. Their rapport has been described as crucial, especially with a U.S. administration that has at times threatened to reduce its military support for NATO. Recently, Secretary of Defense Pete Hegseth announced a six-month review of U.S. troop deployments in Europe, while the U.S. plans to withdraw a significant portion of its fighter jets from NATO emergency readiness—a development Mr. Rutte is expected to address in upcoming talks.

However, this close relationship has drawn criticism. Some European and U.S. officials accuse Mr. Rutte of excessive accommodation toward Mr. Trump, especially on matters outside NATO’s direct scope. His commendation of Mr. Trump’s decision to engage in the war with Iran has particularly raised concerns, as many European leaders regard that conflict as illegal and unrelated to NATO’s responsibilities. Julianne Smith, the U.S. ambassador to NATO under President Joe Biden, questioned the appropriateness of such praise, reflecting broader unease within the alliance.

Mr. Rutte’s supporters argue that his approach is a calculated political strategy aimed at preserving U.S. commitment to transatlantic security, encouraging European partners to increase defense spending, and sustaining support for Ukraine against Russian aggression. His efforts have been credited with defusing moments of crisis, such as during the 2018 summit when Mr. Trump threatened to leave the alliance. Yet, critics caution that this willingness to placate Mr. Trump risks weakening NATO’s internal consensus and credibility, particularly regarding its mutual defense guarantee under Article 5.

As NATO faces ongoing pressures from shifting U.S. policy and global security challenges, Mr. Rutte’s balancing act—promoting alliance unity while managing a difficult relationship with one of its most influential members—continues to draw both praise and scrutiny within the transatlantic community.