The possibility of the US attacking a fellow NATO member, like seizing Greenland from Denmark, has shocked the military alliance. NATO’s key rule, Article 5, says an attack on one is an attack on all. But what happens if the US attacks a member? Denmark’s Prime Minister Mette Fredriksen said on Monday, "If the US chooses to attack another Nato country, everything will stop." This threat raises big questions about NATO’s strength and future. During the 2024 US election, Donald Trump said he would not defend "delinquent" NATO countries that don't spend enough on defense. His defense secretary, Pete Hegseth, said the US is no longer mainly focused on protecting Europe. Despite NATO talks and promises to raise defense spending to 3.5% of GDP by 2035, tensions remain. Marion Messmer from Chatham House said, "Yes, the summit went well in that Rutte found formulations that flattered Trump. But I'm not sure how far that is a sustainable strategy." The capture of Venezuela’s Nicolás Maduro by US forces showed America’s huge military power. Stephen Miller, a Trump adviser, said that the world is "governed by strength, that is governed by force, that is governed by power" – not by treaties. The US has 1.3 million troops and plans $845 billion defense spending in 2025, while Denmark has 13,100 troops with far smaller funds. No NATO rule clearly explains what happens if one member attacks another or takes territory. Still, if the US seizes Greenland, NATO’s credibility will suffer badly, especially with growing threats from Russia. Messmer warns, "If any European states harbour any illusions they can rely on US security guarantees, then this is the wake-up call we are not returning to that world."