From Wikipedia

Oh Fatal Ambition  1777-1778

The capture of a British army at Saratoga encouraged the French to formally enter the war in support of Congress, as Benjamin Franklin negotiated a permanent military alliance in early 1778, significantly becoming the first country to officially recognize the Declaration of Independence. William Pitt spoke out in parliament urging Britain to make peace in America, and unite with America against France, while other British politicians who had previously supported independence now turned against the American rebels for allying with a formerly mutual enemy.

Later Spain (in 1779) and the Dutch (1780) became allies of the French, leaving the British Empire to fight a global war alone without major allies, and requiring it to slip through a combined blockade of the Atlantic. The American theater thus became only one front in Britain's war. The British were forced to withdraw troops from continental America to reinforce the sugar-producing Caribbean islands, which were considered more valuable.

Because of the alliance with France and the deteriorating military situation, Sir Henry Clinton, the British commander, evacuated Philadelphia to reinforce New York City. General Washington attempted to intercept the retreating column, resulting in the Battle of Monmouth Court House, the last major battle fought in the north. After an inconclusive engagement, the British successfully retreated to New York City. The northern war subsequently became a stalemate, as the focus of attention shifted to the smaller southern theater.