[Juan] Guaido and his supporters — which includes the U.S. and some 50 outside nations — had amassed food and medicine at border points into Venezuela from Colombia and Brazil. The aid was meant both to salve a once rich nation brought to poverty under the autocratic Maduro and to show Guaido, 35, proclaimed rival president, as legitimate heir and, in some sense, savior.

But Maduro used his still-loyal military, the nation’s most powerful institution, to crush the effort. As many as four people were reported killed and 200 wounded. Meantime, about 100 soldiers defected. Little to no aid crossed the border. ...

Guaido, who is head of the National Assembly, which named him president after Maduro stole national elections last year, faces problems of his own. He holds virtually no power inside Venezuela, and is now in Colombia and faces a challenge to return after Maduro banned him from travel.