I have always wondered what divided Germans so quickly during post ww2 to the point that they would work border patrols to actively stop their countrymen from fleeing.
More can be written, but as for the "how" part, you might like to start with some previous discussions.
For the Berlin Wall, "Was the Berlin wall a wall around just West Berlin?" by /u/kieslowskifan.
For other borders, you can see "Why did people try to defect over the wall in Berlin and not through some unguarded part of the huge border between East and West Germany?". /u/Poebbel covers the "Inner Border" fortifications, they and /u/Brickie78 covered travel through other countries, and ChuckNorrisAteMySock mentioned naval forces.
As for "why", you might like "why was the berlin wall built?" by kieslowskifan.
This is not to discourage discussion. More questions, data and debate are welcome.