He Previously Served Time in Israeli PrisonIn May 1989, Israel arrested Deif, shortly after Yahya Sinwar's arrest that same year. Soon after his release, he continued advancing within Hamas's military structure, joining the Qassam Brigades in 1991. He actively participated in the project to manufacture Qassam rockets, working alongside Adnan al-Ghoul, considered Hamas's "chief scientist." After the elimination of Salah Shehadeh in July 2002, Deif assumed leadership of Hamas's military wing—a position he held for more than two decades. Deif survived that attack as well, though he was injured and subsequently suffered from speech difficulties and limited limb movement.