The number of evacuees increased greatly in 1945; historian Thomas R.H. Havens has written that the movement of Japanese civilians from cities in the last months of the war was "one of history's great migrations".[6] Following the firebombing of Tokyo on 9–10 March 1945, all schoolchildren in the third to sixth grades were required to leave the main cities, and 87 percent of them had been moved to the countryside by early April.