U.S. President Donald Trump has threatened to leave NATO (NATO), a non-American alliance that includes European countries, the United States and Canada, and other allies. The move could exacerbate tensions with allies, with experts warning that it could encourage Russia to test NATO members' readiness to enforce the alliance's Article 5, which states that an attack against one member state is an attack on all. France was among the first European NATO members to respond, though without directly addressing the threat. Germany remained committed to NATO, while France refused to allow Israel to use its airspace to resupply a flight carrying American weapons used in the war against Iran. Meanwhile, Europe is watching Trump's efforts to broker an end to the war between Russia and Ukraine, with some senior European officials concerned that he appears to support an agreement in Moscow's favour.