vintage postcards
In America, the first flip flops started to appear after World War II as soldiers brought Japanese zori back from the war as souvenirs.
The Nihon fuuzokushi jiten (Encyclopedia of the History of Japanese Manners and Customs) reports the wide use of zori in Japan from at least the Heian period (794-1185). Japanese children have traditionally worn them when learning to walk. (This is understandable because I always trip in mine... what a learning curve)