After rejoining the army at the start of World War II, he was captured (1940), escaped from a German prisoner-of-war camp (1941), and fought with the Allied forces in North Africa, Italy, the south of France, and finally in the invasion of southern Germany.

He ended the war in command of the 3rd Moroccan Spahis (mechanized cavalry), whose distinctive red cap he subsequently wore throughout his service in French Indochina.

In 1946, de Castries, soon to become a lieutenant colonel, was sent to Indochina where he commanded a tabor (battalion) of Moroccan goumiers.

After an eight-week siege, the garrison was defeated, and near the end, de Castries was promoted to brigadier general.

Upon his return to France, de Castries was appointed to command the 5th Armored Division, then stationed in West Germany.