Diaries in WWII
In the Pacific Theater of WWII, many Japanese soldiers were killed, but some were captured alive.
But it was not only from Japanese prisoners of war that the US military got precious information.
Most of Imperial soldiers and officers of Japan kept a diary even in a battle field. After a fierce battle was over and Japanese troops were driven out, US soldiers picked up many diaries and other documents from corpses of Japanese and from inside trenches.
Those diaries gave precious military information to US forces. Americans could understand how Imperial forces were organized and how their bases were fortified in addition to minds and heats of Japanese soldiers.
In order to read and translate these documents and inquire Japanese prisoners of war, the US Government recruited a large number of second-generation Japanese Americans and other Americans who were learning Japanese in universities and so on.
In the Imperial Army, Japanese soldiers and officers were advised to keep a diary. It was probably regraded as a kind of discipline. Soldiers wrote many things almost freely without inspection by supervisors.
Especially in lines of battle in the Pacific Ocean, the Imperial Army did not mind if those diaries could be taken by Americans. Japanese generals thought that Americans could not read and understand diaries Japanese wrote, since the written Japanese language was complicated and difficult. They even did not strictly control handling of military documents so as not to leave them behind when Japanese troops retreated. No Japanese elites even dreamed that the US military was mobilizing thousands of second-generation Japanese Americans and other Americans who could read and speak Japanese to get information in front lines.
Of course, in addition to Americans capable of reading Japanese, many Japanese prisoners of war cooperated in reading and interpreting diaries in addition to providing information they had. (However, it is also true that many Japanese soldiers and officers chose to commit a suicide before being taken captive by US or other allied forces.)
During WWII, the US had not CIA, but their experiences in the war against the Empire of Japan gave precious experiences in collecting information from an enemy whose language and culture were so different from those of the US.
Anyway, even today, many Japanese like to keep a diary. In primary school in Japan, teachers always teach pupils how to keep a diary. And they show good examples of a diary.
In addition, it was expected that 100% of Japanese soldiers could read and write during WWII as the literacy rate of the Empire of Japan was almost 100% far exceeding the wold average at the time.
What's more, Korea before annexation to the Empire of Japan in 1910 had only 10% of the literacy rate. But 25 years after the annexation, the literacy rate among Koreans increased to 65% in 1936. The Empire of Japan saved and educated the Korean people who had been abandoned by their royal government before the annexation to the Empire.
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Luk 10:3 Go your ways: behold, I send you forth as lambs among wolves.