Answer:
The following are the four German actions bringing European tension to the breaking point:
German involvement in the Balkans.
German friction with Serbia
Proposed German railroad.
Build-up of Germany navy.
Step-by-step explanation:
These four actions made by Germany after World War I were responsible for increasing European tensions to the breaking point, which ended up with the beginning of World War II. After World War I, European nations were afraid that Germany would rise up again and start a new set of conflicts, and with the rise of Hitler and the Nazi Party in Germany, the fears started to materialized. Germany felt a sense of revenge for the sanctions the Treaty of Versailles had imposed on it, and with actions such as the build-up of the country's navy, the extension of its railroad system, and with friction and involvements such as the Balkans and with Serbia, the rest of the European nations started to feel an increasing tension towards Germany and its leaders.