BUENOS AIRES, Argentina — When far-right economist Javier Milei was inaugurated as Argentina’s new president last month, his supporters thronged the streets of the capital, Buenos Aires, bearing the national flag and donning Messi soccer jerseys. Scattered among the sea of light blue and white were a handful of Venezuelan flags — a recurring sight in Milei campaign gatherings. Holding one of them was Laura Ruiz. When Milei emerged from the presidential palace to give a speech, Ruiz waved her flag with gusto. Her hope, she said, was for Milei to see the flag — and to know that Venezuelans...