In the period from the Treaty of Frankfurt in 1871 to the outbreak of World War I, the European powers were ostensibly at peace but locked in a fierce rivalry which found expression in the New Imperialism, a period of rapid colonial expansion. Spurred by a second industrial revolution, the appetite for new markets was accentuated by the Long Depression of 1873–96. An intellectual rationale mixed prevailing theories of Social Darwinism with notions of the ‘White Man’s Burden’, holding it to be both natural law and just to conquer and civilize the ‘weaker races’ (echoed in The United States’ westward pursuit of ‘manifest destiny’). Revolutions in medicine, weaponry and transportation made it feasible for the first time to occupy vast tracts of previously impenetrable territory, for instance, in the ‘Scramble for Africa’. The advent of new rival colonialists – Germany, Italy and Japan – only served to accelerate the process.