Franz Halder in the context of "Erich von Manstein"

The Manstein plan or Case Yellow (German: Fall Gelb; also known after the war as Unternehmen Sichelschnitt a transliteration of the English Operation Sickle Cut), was the war plan of the German armed forces (Wehrmacht) for the Battle of France in 1940. The original invasion plan was an awkward compromise devised by General Franz Halder, the chief of staff of Oberkommando des Heeres (OKH, Army High Command) that satisfied no one. Documents with details of the plan fell into Belgian hands during the Mechelen incident on 10 January 1940 and the plan was revised several times, each giving more emphasis to an attack by Army Group A through the Ardennes, which progressively reduced the offensive by Army Group B through the Low Countries to a diversion.

In the final version of the plan, the main effort of the German invasion was made against the Ardennes, the weakest part of the Allied line, where the defence was left to second-rate French divisions in the Second Army and the Ninth Army, on the assumption that the difficulty of moving masses of men and equipment would give the French plenty of time to send reinforcements if the area was attacked. The Seventh Army, which had been the most powerful part of the French strategic reserve, had been committed to a rush through Belgium to join with the Dutch Army to the north, in the Breda variant of Plan D, the Allied deployment plan.

Reinhard Gehlen (3 April 1902 – 8 June 1979) was a German military and intelligence officer, later dubbed "Hitler's Super Spy," who served the Weimar Republic, Nazi Germany, and West Germany, and also worked for the United States during the early years of the Cold War. He led the Gehlen Organization, which worked with the CIA from its founding, employing former SS and Wehrmacht officers, and later became the first head of West Germany's Federal Intelligence Service (BND). In years prior, he was in charge of German military intelligence on the Eastern Front during World War II and later became one of the founders of the West German armed forces, the Bundeswehr.

The son of an army officer and World War I veteran, in 1920 Gehlen joined the Reichswehr, the truncated army of the Weimar Republic, and was an operations staff officer in an infantry division during the invasion of Poland in 1939. After that he was appointed to the staff of General Franz Halder, the Chief of the Army High Command (OKH), and quickly became one of his main assistants. Gehlen had a significant role in planning the German operations in Greece, Yugoslavia, and the Soviet Union. When the Red Army continued to fight after the initial German success during Operation Barbarossa, in the spring of 1942 Gehlen was appointed by Halder as director of Foreign Armies East (FHO), the military intelligence service of the OKH tasked with analyzing the Soviet armed forces. He achieved the rank of major general before he was dismissed by Adolf Hitler in April 1945 because of the FHO's alleged "defeatism" and accurate but pessimistic intelligence reports about Red Army military superiority.