US President Donald Trump's rhetoric on everything from punitive tariffs to "ownership" of Greenland has been prompting a rethink of Europe's defense strategy. In Brussels, diplomats, officials and think-tankers have spent much of the past year debating whether the US would still respect Article 5 of the North Atlantic Treaty Organization (NATO), which pledges that an attack on one signatory state would be seen as an attack on all, if called upon.