During World War II Battle of the Atlantic, the Allies realized that to combat the threat posed by the U-boats a more aggressive strategy was needed than simply providing convoys with escorts. Advances in signals intelligence such as 'Huff-Duff' (HF/DF), in crypotological intelligence such as Ultra, and in detection technologies such as radar and sonar/ASDIC enabled the Allied navies to form flotillas designed actively to hunt down submarines and sink them. A Hunter-killer Group would typically be formed around an escort carrier (CVE) to provide aerial reconnaissance and air cover, with a number of destroyers, destroyer escorts, frigates, and/or Coast Guard cutters armed with depth charges, K-guns and Hedgehog.