When England was conquered by the Normans in 1066, it was profoundly changed. We might expect the same to have been true in Japan's case if it had fallen to the invading Mongols and their Chinese and Korean auxiliaries in 1274 or 1281.
Two things prevented Mongol success. One was the famous kamikaze typhoons, which each time wrecked large parts of the invading fleets. The other was the unyielding resistance of the Japanese and their leader Hojo Tokimune (1251-1284). To coincide with the current NHK Sunday night historical series on Tokimune, the Edo Tokyo Museum is staging an exhibition of Kamakura artifacts focused on the heroic events of his life.
The scrolls and documents on display include everything from official and diplomatic correspondence to diaries and poems. Among them is a copy of the letter sent from the court of Kublai Khan in 1268 demanding tribute from the smaller island nation.