When the Islamist group Hamas conquered the Gaza Strip in June it seized an intelligence-and-military infrastructure created with U.S. help by the security chiefs of the Palestinian territory's former ruler.

According to current and former Israeli intelligence officials, former U.S. intelligence personnel and Palestinian officials, Hamas has increased its inventory of arms since the takeover of Gaza and picked up technical expertise -- such as espionage techniques -- that could assist the group in its fight against Israel...