In June 1779, Spain entered the war against Britain on the side of its ally, France and its ally the nascent United States of America. Although logistical aid by the French and Spanish for the American cause were both immense and essential, the French military assistance was plagued with problems compared to the Spanish. Throughout 1779 and 1780, tangible Spanish victories against the British in support of the Americans greatly outnumbered French. Whereas French operations against Newport and Savannah were costly failures, the Spanish quickly defeated the British in West Florida and in the Mississippi valley, greatly assisting the Americans in securing the future Northwest Territory. Furthermore, most supplies destined for the Americans arrived through Spain’s Gulf Coast and Spanish ports in the Caribbean and West Indies became havens for American privateers.

For operations in the Caribbean and Gulf of Mexico against the French and Spanish, a British convoy set sail from Portsmouth in early August 1780 for the West Indies carrying critical naval stores and the 90th Regiment of Foot. The enormous convoy consisted of five East Indiamen, massive merchantmen used by the British East India Company in the Pacific and Indian Oceans, 55 smaller West Indiamen, and escorted by the full might of the British Channel Fleet.

On word from Spanish spies of the convoy’s departure, Spanish Admiral Luis de Córdova sortied from Cadiz with 31 ships of the line and six frigates to intercept. Though not enough to challenge Britain’s Channel Fleet, Córdova bet that the British would not escort the convoy far leaving the Channel unguarded. He was correct. On 2 August, 1780, the Channel Fleet returned and left just one ship of the line, the HMS Ramillies, and two frigates as an escort for the convoy. When the fleet departed, the convoy came under command of Admiral John Moutray on the Ramillies.
Moutray wasn’t expecting any French or Spanish interference, and if there was, he was confident that he could out maneuver any attempt to capture the convoy. Moutray was wrong on both accounts. Córdova intercepted Moutray off of Cape Saint Vincent on the evening of 8 August. Unperturbed, Moutray signaled the convoy to turn, which would have taken the convoy to safety by the next morning.

Unfortunately for Moutray, Córdova got close to the convoy that night, so close in fact that Córdova signaled some of the British merchantmen to follow him. Mistaking Córdova’s lanterns for Moutray’s, the vast majority of the convoy followed and turned away from their escorts and safety. In the early morning hours of 9 August 1780, 55 British merchantmen, including the five giant East Indiamen, were interspersed among the Spanish Fleet. At dawn, Córdova signaled a general chase. Surprise was complete.
After a brief action, the Spanish captured all 55 British ships, with only the ships that followed Moutray, the escorts and eight West Indiamen, escaping. Cordova captured 3100 British sailors, civilians, marines, and soldiers, including the entire 90th Regiment of Foot. He also took £1.5 million in gold, supplies, muskets, cannon, powder, and naval stores (worth about $350 million today), much of which was used by the Continental Army and Navy against the British in North America. The East Indiamen were incorporated into the Spanish Navy. The Action on 9 August 1780 was one of the costliest failures of the British intelligence apparatus in its history. Córdova’s victory bankrupted most British maritime insurance companies, whose premiums were already high due to losses by privateers. The bankruptcies caused an uproar in London’s financial district which soon spread to Parliament. The uproar greatly contributed to the growing movement to end the stalemated war against their former colonies in North America.