In 1940 Moussaieff joined the Etzel which opposed British rule in Palestine. Upon the recommendation of his Etzel leader, he joined the British Army at age 17 to fight Nazi Germany during World War II. Stationed in the Egyptian desert and Livorno, Italy, he searched through synagogue "genizot" (treasuries) during his free time and bought old Kabbalah manuscripts and marriage contracts written by well-known rabbis. In 1947 he rejoined the Etzel to battle the Arab Legion in the Old City of Jerusalem. When the city fell to the Jordanians in 1948, he was taken captive and imprisoned for one year in Transjordan. He married his wife, Alisa, an Austrian native, two weeks before he went into captivity.