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After World War II、many Amecrian Soldiers spread Instant coffee like Nescafe、and it was included in Care packages delivered to civilian populations around in Western Europe, Japan right after WWII and so on.
But in Japn, instant coffee is not spread soon, because of limitation of import of coffee regulation.

After that, in the 1960s, when restrictions on imports of instant coffee were lifted in Japan, instant coffee became popular.

By the time the U.S. entered World War II in 1941, Nescafé was already in the emergency rations of every departing U.S. soldier , so the hundreds of thousands of American soldiers who occupied Japan from 1945-1952 would have been familiar with it and likely brought it with them.
However, coffee didn't immediately become popular in Japan. During World War II, the Japanese government established a ban on coffee imports that lasted until 1950 , which limited its availability during the early occupation period.