Final answer:
There seems to be a mix-up of dates as no direct significant historical event aligns with September 12, 1938; the closest related event is the Munich Agreement announced by British Prime Minister Neville Chamberlain on September 30, 1938.
Step-by-step explanation:
The inquiry about the significant event that took place on September 12, 1938, seems to be a mix-up of dates and events. However, it is closely related to important occurrences in the late 1930s involving Germany. A key historical milestone was British Prime Minister Neville Chamberlain's Munich Agreement announcement on September 30, 1938, not September 12, following his return from Berlin.
Nonetheless, the provided information erroneously combines several historical events, leading to confusion regarding the exact event on September 12, 1938. The collapse of the Mark in 1923, Hitler's Beer Hall Putsch, and the subsequent writing of Mein Kampf all occurred in the early 1920s, and are not directly linked to the specific date in question.