Spain was not a bystander to the American Revolutionary War, although that fact is rarely mentioned in cursory historical surveys. Spain's motivation to help the American colonists was driven by a desire to regain the land it had lost to Britain and, with other European powers, make incremental gains against British possessions in other parts of the world. Although some dreamers in Spain perhaps envisioned its eventual possession of the entire New World, I have found no evidence that such an idea guided its assistance to the American colonists.

France and Spain were at that time both under Bourbon kings, Louis XVI and Carlos III, respectively, whose American possessions had been significantly reduced by the 1763 Treaty of Paris that ended the Seven Years' (the French and Indian) War. At the beginning of the American War of Independence, American commissioners were sent to Europe by the Continental Congress to seek support for their cause. John Jay, American representative in Spain, found success. Americans promised both France and Spain the restoration of much of the land they had lost to the British in America. In April 1779, Spain committed to helping the Americans.

This help did not consist of Spanish troops to fight alongside Americans, but it was extensive nevertheless. The Spanish and French kings provided large loans and outright contributions of money to the Americans. Spain laundered this money, as we would say today, through a fictitious private trading company, Roderique Hortalez and Company, operating out of the Lesser Antilles, which sent both money and war material directly to the Americans. The money helped support the Americans' new currency, the Continental, and also made it possible for the Americans to bring in foreign military officers, such as Augustus von Steuben, Casimir Pulaski, and Thaddeus Kosciuszko, to fight for them.

Spain began a military campaign of its own against the British in Florida and Louisiana. From 1779 through 1782, the Spanish Governor of Louisiana, Don Bernardo de Gàlvez, conducted a series of military actions against the British to retake forts that Spain had earlier lost to the British, succeeding in the Mississippi River Valley, and at Baton Rouge, Natchez, Mobile, and Pensacola. In 1782, Spain also succeeded in wresting back the Bahamas from the British.

A very substantial form of Spain's support for the Americans involved a strategy of joining Britain's other European competitors in tying up British naval resources by engaging them elsewhere than in Britain's American colonies. Spain did this, for example, against Gibraltar and Minorca, and together with France sent a fleet into the English Channel to menace the British coast and tie up more British ships. Most of the European maritime powers, including Spain, united against Britain's effort to interrupt their trade with America. With both France and Spain (and Holland) indirectly in the fray, Britain's navy was outmatched and could not effectively concentrate its military force in America. Spanish ships joined with French ships in the naval blockade of the British army at Yorktown in 1781, preventing General Cornwallis's resupply by the British navy, resulting in his surrender.

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After Spain's public acknowledgment in June 1779 that it would join with France to wage war on Britain, the Governor General of Spanish Louisiana, Bernardo de Gálvez, began his successful military campaign against British forces in Florida, Louisiana, and in the Mississippi River Valley.

John Jay's mission to Spain, which began in September of that year resulted in no additional direct aid to the American colonies, although Spain funneled some money to them indirectly, which helped the Colonies stabilize its currency. But Spain had had reasons of its own to harass Britain militarily and it had begun to do just that, conscious that Britain's world-wide military power would be occupied to some extent by the revolt of its colonies in America.

The Catholic monarchy of Spain, however, had little sympathy with the British colonies' budding republican ideals. The U.S. State Department's website, describing Jay's mission to Madrid, says that Carlos III's minister, with whom Jay dealt, "worried about American claims to lands west of the Appalachians and navigation rights on the Mississippi River and feared that the flames of the American revolution might spread to Spanish colonies in the Americas."

Nevertheless, his worry did not hinder Spain's pursuit of its own interests in America against the British and this certainly had the indirect but substantial effect of supporting the American colonists.