US, Sept. 17 -- As Trump threatens Europe over its extraordinary invitation to Canada, a geopolitical dispute could have an unexpected consequence: making it easier for Canadians and Europeans to travel, study, work and do business across the Atlantic.

Canada becoming an "associate member" of the European Union sounded almost unimaginable until European Commission President Ursula von der Leyen publicly opened that door in Strasbourg.

Now U.S. President Donald Trump has made sure the proposal cannot be dismissed as another obscure piece of European diplomacy.

Trump called the idea "laughable" and warned that if he considers closer EU-Canada association a hostile act, Washington could impose "very serious" tariffs or restrict trade with...