How are country borders decided to begin with and how are they enforced? What events decides/decided how large a country is/was? Also, I a friend was talking me about Mittelafrika and he said that some African borders were simply determined by colonizers and that's a reason why there are so many civil wars in Africa. Can fellow historians of reddit elaborate on the matter?
In Europe, the present-day borders are the result of a long historical process. Some of them (usually the ones that correspond to natural markers, e.g. the Pyrenees) have been around since prehistory. Many others were decided on by one of the conferences that followed major wars during the last 200 years or so. The Vienna Congress of 1815, which dissolved the Empire of Napoleon, created a large part of the international borders that exist in western Europe today. But of course they did not just draw random lines -- they based the new borders on borders of subnational states that had existed before.
The last time completely new borders were drawn in Europe was immediately after World War II, e.g. by the Potsdam Conference of 1945. Germany was split up and the western border of the Soviet Union was redrawn. Since then, some countries dissolved into their constituent parts (e.g. Yugoslavia), but no major new borders were drawn.