The boundary between British and French possessions in North America was largely undefined in the 1750s. France had long claimed the entire Mississippi River basin. This was disputed by Britain. In the early 1750s the French began constructing a chain of forts in the Ohio River Valley to assert their claim and shield the Native American population from increasing British influence.
The most important French fort planned was intended to occupy a position at "the Forks" where the Allegheny and Monongahela Rivers meet to form the Ohio River (present-day Pittsburgh, Pennsylvania). Peaceful British attempts to halt this fort construction were unsuccessful, and the French proceeded to build the fort they named Fort Duquesne. British colonial militia from Virginia accompanied by Chief Tanacharison and small number of Mingo warriors were then sent to drive them out. Led by George Washington, they ambushed a small French force at Jumonville Glen on 28 May 1754 killing ten, including commander Jumonville. The French retaliated by attacking Washington's army at Fort Necessity on 3 July 1754 and forced Washington to surrender. These were the first engagements of what would become the worldwide Seven Years' War.
News of this arrived in Europe, where Britain and France unsuccessfully attempted to negotiate a solution. The two nations eventually dispatched regular troops to North America to enforce their claims. The first British action was the assault on Acadia on 16 June 1755 in the Battle of Fort Beauséjour, which was immediately followed by their expulsion of the Acadians. In July British Major General Edward Braddock led about 2,000 army troops and provincial militia on an expedition to retake Fort Duquesne, but the expedition ended in disastrous defeat. In further action, Admiral Edward Boscawen fired on the French ship Alcide on 8 June 1755, capturing it and two troop ships. In September 1755, British colonial and French troops met in the inconclusive Battle of Lake George.
The British also harassed French shipping beginning in August 1755, seizing hundreds of ships and capturing thousands of merchant seamen while the two nations were nominally at peace. Incensed, France prepared to attack Hanover, whose prince-elector was also the King of Great Britain and Menorca. Britain concluded a treaty whereby Prussia agreed to protect Hanover. In response France concluded an alliance with its long-time enemy Austria, an event known as the Diplomatic Revolution.