King Louis XVI of France formally declared war against Great Britain in July 1778, motivated by a range of geopolitical factors and the desire to weaken British power. France had been secretly aiding the American colonies since 1776, but the declaration marked a significant escalation in their involvement. It was driven by a combination of a desire to restore national honor after the losses in the Seven Years' War and a strategic calculation to undermine Britain, France's long-standing rival. The French monarchy sought to support the American revolutionary cause that was gaining momentum, and thus help ensure a shift in the balance of power in favor of the Americans.

The impact of France’s declaration was pivotal. It changed the dynamics of the American Revolutionary War by not only granting the colonies a powerful ally but also introducing major French military resources and tactics into the conflict. France's support included naval power, troops, and supplies, which played a crucial role in key battles. The French entered the war with seasoned military leaders and a determination to strike against British naval supremacy, significantly altering the course of the war.

Moreover, the Franco-American alliance forged through this declaration set the stage for future diplomatic relations and military cooperation. The assistance provided by France culminated in decisive victories such as that at the Siege of Yorktown in 1781, leading to the eventual British surrender and the recognition of American independence. This not only secured the victory for the American colonies but also re-established France's position as a prominent global power.