Germany, Austria, Switzerland and Liechtenstein. All these 4 countries speak German as their first and official language. they are bordering each other. They once used to be one country, yet they are divided. Always thought about that..
History happened?
First, Switzerland has not one official language but four (in addition to German, it has French, Italian and Romansh).
For Austria, the story is decisively more complicated. Austria used to be part of the Holy Roman Empire (until the Napoleonic Wars, when the HRE was dissolved), and at that point already it was a major power in its own right. When Napoleon dissolved the HRE, Austria declared itself as its own 'empire'. More importantly, Austria (or Austria-Hungary after the compromise) was a multi-ethnic empire, and while speakers of German had the plurality (read: they were the largest single language group), by a large margin they had no majority. To the aspiring thinkers in Germany who wanted to unify the German states after the Napoleonic Wars in the early 19th century (culminating in the failed 1848 revolution), this was a bizarre problem, and they came up with two concepts to solve this: either a 'little' Germany (German
Kleindeutschland) that excluded Austria, or a 'greater' Germany (German
Großdeutschland). The actual unification of Germany under Prussian yoke in the early 1870s was a case of the former. Then World War One happened, and in the end Austria-Hungary was dissolved. The German-speaking rump state (which became modern-day Austria) was forbidden by the winners of World War One (the Entente Powers) from joining Germany. Then in 1930s the Nazis rose to power in Germany, and in the late 1930s, thanks to the dilettantish negotiations of the British (under Neville Chamberlain), Hitler was able to annex Austria and he proclaimed a "Greater German Empire" as a result. Then, after also annexing the German-speaking parts of Bohemia (the so-called "Sudetenland"), Hitler started World War II by invading Poland. After World War II, Austria was restored as a separate state, and it has been separate since. When the Berlin Wall fell in 1989 and negotiations of a reunification between east and west Germany began, there wasn't even a debate about Austria joining Germany. Realistically, if you disregard the period from just before to the end of World War II (1938-1945), Austria has been a separate state that developed separately from the rest of Germany for over 200 years.
And now, cynically, let me ask you a question: why does the United States not rejoin the United Kingdom?