Why did Spain and France get involved in the American Revolution?
Splitting off the Thirteen Colonies would weaken Britain greatly thus improving their positions on the world stage. With British troops occupied trying to suppress the rebellion, there was the possibility that France and Spain could make territorial gains elsewhere in the world.
France hated the British with a passion, almost to a fault. They felt anything they could do to harm Britain would be good for them. The French financial assistance during the Revolution bankrupted them but they continued to feel it would be worth it as revenge for the seven years war. Spain's help was very late in coming and negligible when it arrived. The main if not only reason for Spain to get involved was to regain Gibraltar and keep the areas of North America they controlled (Florida for one) from falling into British hands.
Spain's help for the American colonists was not late and most importantly under no circumstance negligible, if Spain did not help France and the American colonists, the British would have surely defeated the Independence movement of the thirteen colonies. Spain was ask by the king of France to form an alliance against Great Britain. The French king was very aware that he needed Spain and the Spanish colonies to help them defeat Great Britain. Spain's colonies of Mexico, Cuba, Venezuela, Puerto Rico, Dominican Republic all helped tremendously in sending soldiers, sailors, war materials as well as a great deal of monetary help for the cause of the American Revolution.
Years before Spain declared war on Great Britain in 1779, the great Spanish General Bernardo De Galvez of Spain were providing the American colonists along the entire length of the Mississippi River from the Gulf of Mexico to as far north as Canada with food, medicine, guns/cannons, gunpowder, uniforms and money. Bernardo de Galvez defeated the British all along the Mississippi River at various British forts; pushing the British out of the lower half of the North American continent. The Battle of Pensacola was one of the most hard fought important battles of the American Revolution and it was de Galvez who defeated the British forces at the Battle of Pensacola. Therefore, with these Spanish victories, the British were prevented from attacking the American colonists from using the western and southern fronts of North America as a staging ground to attack the thirteen colonies.