These stories take place in the first half of the 20th century, between the years 1900-1949. Notable events from this time period include the invention of the airplane, both world wars (including the Holocaust and bombing of...
These are manga that take place in Japan during the Showa era (1926-1989). Most (though not all) of these manga focus on WWII, it's aftermath, or use it as a backdrop of some sort.