The British had caught wind of the commercial tensions in Buenos Aires, mistakenly interpreting them as revolutionary. In June 1806 , a force of 1600 men led by General William Beresford stormed into the city unchecked. Beresford hoped, ultimately, to assert British imperial control over the entire Viceroyalty and bring it into the British trading orbit. The Viceroy, the Marqués de Sobremonte, reacted to the news of the British landing by fleeing the city and the Spanish authorities grudgingly swore allegiance to the British Crown. Among the ordinary inhabitants, there were shouts of "treason" and a sense of offended honour at the way in which such a tiny force had been allowed to overrun the city's defences.

The people of Buenos Aires regrouped under their new commander-in-chief, the French-born Santiago Liniers, and ousted their invaders during the Reconquista of August 12. Undaunted, the British captured Montevideo from where they launched a second assault on a better-prepared Buenos Aires in July 1807. This battle led to the surrender of the British and came to be known as La Defensa , a name imbued with the bravura of Liniers' hastily assembled militia, whose cannon- and musket-fire peppered the enemy, while women poured boiling oil from the tops of the city's buildings on the hapless British soldiers.

One consequence of the victory over the British was to make the people of Buenos Aires aware of the extent to which they could manage their own affairs and how little they could rely on either the viceregal authorities or the motherland. This was the first time that they had fought in unison against a foreign invader and the feeling of pride carried over into a stance of defiance in certain sectors against the monarchy as the Spanish Empire finally started to crumble