(Nearly) a century of circles

One [air raid poster] in particular (above; click for larger version) is very revealing: it shows exactly whose bombers the Japanese were worried about, by plotting circles on a map of Japan and its neighbours, representing the radius of action¹ of bombers from potential enemies. It turns out they were afraid of everybody’s, except for the country they were actually at war with (China). The brown circle shows the radius of action of American bombers from the Philippines; black, British bombers from Hong Kong; green, Russian bombers from Vladivostok; yellow, American bombers from Alaska; and blue is in the middle of the ocean — American carrier-borne bombers, most likely. The circles are marked with a number, probably a distance: 2000 km? That would make some sense, as it was very roughly the radius of action of the B-17s that were just entering service in the US Army in 1938 (though not in substantial numbers until 1941).

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