Did Otto von Bismarck say the following?:
He who is master of Bohemia is master of Europe
This quote is reported as frequently quoted but unverified in Respectfully Quoted: A Dictionary of Quotations (1989), which states:
"It cannot be found in the official writings and pronouncements of Bismarck. It is possible that he said it, and it was passed on orally rather than being recorded, or that he expressed the sentiment in other terms and the idea took this form as others tried to quote him."
I was too slow to compose this answer before the question was edited, but hope it's useful anyway. This addresses why Bismarck may have said this quote.
It's difficult to know Bismarck's intention given that we don't know if he said this at all, but I'd guess it's mostly to do with Bohemia's (and by extension, Czechoslovakia's) economic and industrial prowess. For the rest of this answer I'm conflating Bohemia with Czechoslovakia and examining pre-WWII situations, which I hope will not invalidate my central points. Bohemia was (is?) one of the most well-developed parts of Czechoslovakia after all.
Nazi Germany selected Czechoslovakia as one of its first countries to invade with good reason (probably the first hostile invasion given that Austria was mostly willing). Although Czechoslovakia is not one of the Great Powers, it is not far down the list, and definitely punches above its weight:
- It concentrated 70% of the industry of the former Austro-Hungarian empire
- It was number 10 in terms of global industrial output (remember, not far behind the great powers)
- Skoda Works, located in Pilsen (in Bohemia), was one of the largest in Europe, almost out-producing Britain between late-1938 and late-1939. At one point in 1945, it represented 30% of arms supplies for the German army
- Czechoslovakia had the highest per capita GDP of Warsaw-pact Europe
Given its industrial importance, Bohemia would have been a necessary stepping stone towards European conquest, although I'm not sure its as important as the quote makes it sound.
It's a popular quote often repeated by Czech politicians and intellectuals - for example, Czech president Havel loved to frame the Czech lands in this way.
However, many historians have tried very hard to find the original source of this quote by Bismarck and they have found nothing. The "Bismarck quote" is most likely a myth. The original creator of the myth is probably the French historian of Germany, Ernest Denis, who became a big defender of the Czechoslovak interests when he was deceived by the officials in Vienna and Budapest.
My source was:  - The passage is: Ernest Denis and Seton-Watson became champions of our cause because, before 1914, they have been lied to by authorities in Vienna and Budapest. The assertion that the quote was probably inserted to Bismarck's mouth by no one else than Denis came from this essay:
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