Mexico in 1821 was a nation that did not yet exist. It had fought eleven years for independence, won it on paper, and now faced an enemy that refused to acknowledge the victory: Spain still held the fortress of San Juan de Ulúa in Veracruz's harbour, a Spanish gun pointed at the heart of the new republic's only major port.

The Treaty of Córdoba, signed September 27, 1821, declared Mexico free. Spain rejected it. The Spanish viceroy who signed it, they argued, had no authority to give away a colony. Madrid would not recognise the independence, would not withdraw its troops, and would not stop using Cuba as a staging ground for reconquest. Mexico had won a war but inherited a hostage situation: the fortress held 2,000 Spanish soldiers, supplied from Havana, waiting for orders to retake the country.

For four years the two sides circled each other. The Mexican government built a navy from scratch, acquiring ships from Britain and the United States. Spanish commanders negotiated, stalled, received fresh reinforcements from Cuba. On September 25, 1823, Spain answered Mexican patience with cannon fire: the Spanish bombarded Veracruz itself, killing civilians and displacing over six thousand people from the city. Mexico had to act.

In October 1823, the Mexican Navy began a blockade of San Juan de Ulúa. No supplies in, no reinforcements out. For nearly two years the fortress sat besieged by a navy that barely existed, commanded by officers learning seamanship as they fought. The Spanish garrison, cut off from Havana, grew hungrier and weaker. On November 23, 1825, the Spanish commander surrendered. Mexico's last Spanish stronghold fell.

But Spain would not accept defeat. In 1829, General Isidro Barradas sailed from Cuba with over three thousand soldiers and orders to reconquer the mainland. He landed near Tampico on July 26, 1829, and began marching inland. The Mexican General Antonio López de Santa Anna moved to block him. On September 10 and 11, at Pueblo Viejo, the two armies met.

Barradas's force was trapped between Mexican troops and the swamp. His soldiers were sick, outnumbered, and far from resupply. On the second day of fighting, he signed the capitulation. Spain's last attempt to retake Mexico had failed in forty-eight hours.

Seven years later, on December 28, 1836, Spain finally recognised what it had refused to accept for fifteen years: Mexico was independent. The Santa María–Calatrava Treaty, signed in Madrid, ended the legal fiction that the colony still belonged to Spain. Mexico had defended its independence not through a single decisive battle but through blockade, negotiation, and the exhaustion of Spanish will. A fledgling nation had held off an empire, and Europe had to accept it.