The years 1756 and 1757 brought three things: the arrival of Louis-Joseph de Montcalm, newly appointed commander-in-chief of the French forces in North America, declarations of war by the two mother countries, and a string of French victories in forts along the Northeast frontier.
While General Edward Braddock's defeat at Fort Duquesne was offset by William Johnson's victory at Crown Point, 1756 and 1757 brought nothing but bad news for the English. With the arrival of Montcalm in March 1756, an exceptionally talented strategist and warrior, the French forces gained a new level of professionalism, savvy, and strength. The British, meanwhile, were disorganized and fighting among themselves. Conflicts between British officers and colonial militiamen were common, culminating in the summer of 1756, when the regiments headed to Crown Point were upset by a small "mutiny."
After almost two years of battles, England and France finally declared war on each other in May 1756. The declaration brought an influx of funding colonies and the arrival of even more British troops. The Earl of Loundoun was appointed commander-in-chief of the British troops in America, but he shortly proved himself as inept as Braddock in the all-important areas of Indian policy and frontier battle strategy. It was under Loundoun's command that the "mutiny" of colonial militiamen exploded, and it was under his command that the British suffered some of the worst defeats of the war.
One of most devastating of these defeats was the fall of Fort Oswego on August 14, 1756. The loss of the fort shocked the British, though in hindsight it's fall seems unsurprising. The fort was devastated by long periods of neglect. The surrounding tribes were already hostile to the British, and Montcalm swayed them further to the French side by spreading a rumor of plunder as a reward for all Indians who came to fight. The fort offered little resistance, and it fell to the French easily. This was an important strategic gain for the French, as it offered them control of Lake Ontario and access to all of the provisions and equipment that had been painfully transported to the fort.
The "mutiny" at Crown Point was another example of British failure to think clearly regarding colonial policy. Loundoun humiliated colonial officers by placing ceilings upon their rank, announcing that a regular British captain would outrank even the highest-ranking colonial. Loundoun caused further consternation by ordering that the troops be incorporated into a single body. His intention was clearly to fortify the colonial troops with British men; the colonial men did not take kindly to the implicit assumption of their inferiority. When Loundoun's orders met with resistance, he denounced the colonials as mutinous and sent many of them home.
Both England and France resisted declaring war on each other for as long as possible. To the mother countries, war meant expense, and colonies were meant to be pure profit—a declaration of war would cut into their budgets and make the colonies less profitable, possibly for years to come. The French and English were willing to wage an undeclared and even partially neglected war in North America. However, when Prussia invaded Saxony in 1756, triggering a European war, called the Seven Year's War in which France, Sweden, Russia, and Austria-Hungary sided against the Prussians and English, the pressure became too great: France and England declared war on each other. The Seven Year's War has been called the first modern world war: it encompassed several continents and three separate names. There were campaigns not just in North America (The French and Indian War), but Europe (Seven Year's War), India (Third Carnatic War), Africa, and the Caribbean. Other European countries, including Austria, Russia, Prussia, and Spain were also drawn into the conflict.