World War I
Germany is in the middle of Europe and so it had to worry about enemies to its east and west. Germany's plan was the quick defeat of France in the west before Russia was ready to fight, then Germany would move its armies to the east to face Russia. However, Germany could not quickly invade France directly because France had put so many forts on the border. That made Germany invade the neighboring country of Belgium to invade France through the undefended French-Belgian border.