The power of Huerta was waning perceptibly and that of the Constitutionalists was increasing when an incident that occurred in April, 1914, at Tampico brought matters to a climax.

A number of American sailors who had gone ashore to obtain supplies were arrested and temporarily detained.

The United States demanded that the American flag be saluted as reparation for the insult.

Upon the refusal of Huerta to comply, the United States sent a naval expedition to occupy Vera Cruz.
Both Carranza and Huerta regarded this move as equivalent to an act of war.

Argentina, Brazil, and Chile then offered their mediation.