Final answer:
France allied itself with the Americans after the Battle of Saratoga, which was considered the turning point of the war.
Step-by-step explanation:
The major European power that allied itself with the Americans against the British after the American victory in the battle considered the turning point of the war was France. The American victory at the Battle of Saratoga convinced the French to recognize American independence and form a military alliance with the new nation. The French provided military support to the Americans, including soldiers, naval forces, and military leaders, which played a decisive role in the defeat of Great Britain.