Both Summer and Winter games were cancelled in 1940 and 1944 due to World War II.
Tokyo and Sapporo, Japan, were the venues scheduled for the XII Olympiad (Summer Games) and what would have been the V Olympic Winter Games, respectively, to be held in 1940.
The XIII Olympiad (Summer Games) in 1944 was scheduled for London and had to be cancelled. London was able to host in 1948. The cancelled 1944 Olympic Winter Games had been scheduled for Cortina d'Ampezzo, Italy, which later was the actual host in 1956.