Bogota, Sept. 28 -- The United States moved to revoke the visa of Colombian President Gustavo Petro after he joined a surging pro-Palestinian demonstration outside the United Nations in Manhattan and told American servicemembers to "disobey the orders of Trump." By Saturday, Mr. Petro returned to Colombia unbowed and, in a series of defiant posts, said the decision "shows the US no longer respects international law."

The episode, erupting on the sidelines of the UN General Assembly, jolted one of Washington's oldest security partnerships in the Americas. It also exposed a test of host country obligations and the line between protected political speech and what the State Department labeled "reckless and incendiary actions." The legal back...