President Trump on Monday delivered his sharpest warning yet to Venezuela’s military authorities in an increasingly tense showdown over that country’s crisis, proclaiming they would “lose everything” by remaining loyal to President Nicolás Maduro and refusing to allow in emergency aid stockpiled on the border. Mr. Trump gave the warning in a speech denouncing Venezuela’s brand of socialism to an enthusiastic crowd in Miami that included many Americans of Venezuelan descent who have fled Venezuela or have relatives in the country, once Latin America’s wealthiest but now facing the greatest economic collapse in generations. He spoke five days before a deadline that his administration and the Venezuelan opposition leader, Juan Guaidó, have declared for getting humanitarian aid into the country — a move aimed at weakening Mr. Maduro, who is no longer recognized by the United States and roughly 50 […]