In 1938, after the Sudeten Crisis, Jaeger took part in the German invasion of Czechoslovakia's Sudeten-German areas. With the outbreak of the Second World War, he was deployed in the invasion of Poland. From 1939, Jaeger forged contacts with resistance elements within the Wehrmacht, including Hans Oster, Friedrich Olbricht and Ludwig Beck. In 1940, he participated in the Battle of France where he earned the Knight's Cross of the Iron Cross ) and in 1941, he was deployed in the Russian Campaign.