The Seven Years’ War (1756–1763).
It involved every European major power except the Ottoman Empire. It was fought in Europe, the Caribbean, North and South America, West Africa, the Philippines and India.
It was started by Prussia’s Frederick the Great’s unprovoked aggression on Austria. The conflict split Europe into two coalitions: one consisting of the U.K., Prussia, Portugal, Hanover and, the other, France, Austria, Russia, Spain and Sweden.
The war established Prussia as a great power. France lost Quebec, its position as the strongest military power in Europe and the supremacy of the seas.
It is assumed – this is guesswork – that there were altogether one million casualties.