Behrens graduated from the Submarine School, New London, Connecticut, in 1943, and was assigned to the as gunnery officer, for two war patrols, and was awarded Presidential Unit Citation. He was then assigned to the , as a communications and engineer officer, on four war patrols, and was awarded Navy Unit Citation, Silver Star, Bronze Star with "V", with personal citations for "conspicuous gallantry and intrepidity". He was credited with conceiving electronic slip rings allowing for continuous, non-reversing trainability of submarine sonar, as well as developing the first short form code for use in submarine "wolfpack" communications.