A war in Look to the West.
Building on the colonial tensions caused by the First Platinean War and the clearing of the missions that led up to it, native Indians led by Tomas Katari and Tupac Amaru II rebelled against the Spanish authorities in South America in 1779. Colonial militia failed to defeat them, so Spain deployed full military forces.
The Spanish asked the French for help under the Bourbon Family Compact, but France's Louis XVI dithered over whether to do this or to support the rebels, undermining Spanish rule in South America in an attempt to acquire the colonies for France. The result of this was that a French fleet sailed with imprecise orders and invaded the River Plate when the local Spanish colonials had been expecting to greet their allies. Thus, even as the Indian rebellions seemed to be quashed, the colonials of the Viceroyalty of Peru - fired up by the Criollista movement against dominance by Spanish-born Peninsulares - exploded into a far more serious rebellion of their own.
In 1782 Britain entered the war, with tacit backing from Portugal. An Anglo-American army under George Washington was landed in the Plate and helped to defeat the French, forcing a surrender of troops led by the Duc de Noailles who were occupying Buenos Aires. At sea, the Royal Navy suffered a shock defeat at the Battle of Trafalgar and lost Minorca to the Spanish, leading to a reactive buildup of the fleet in the years following the war. However, the Royal Navy nonetheless inflicted enough damage to prevent the Franco-Spanish from resupplying their troops in South America.
The resulting Treaty of London led to the independence of the Viceroyalty of Peru (later to become the United Provinces of South America) with autonomous Indian states within it, and the Empire of North America acquiring the largely uninhabited northern parts of French Louisiana. In a more general sense, it helped spark the later French Revolution, partly by its radical ideology and partly because it bankrupted the French treasury and stoked popular discontent.