During World War II

World War II

World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

 Battle of the Atlantic, the Allies realized that to combat the threat posed by the U-boat

U-boat

U-boat is the anglicized version of the German word U-Boot , itself an abbreviation of Unterseeboot , and refers to military submarines operated by Germany, particularly in World War I and World War II...

s 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

Ultra

Ultra was the designation adopted by British military intelligence in June 1941 for wartime signals intelligence obtained by "breaking" high-level encrypted enemy radio and teleprinter communications at the Government Code and Cypher School at Bletchley Park. "Ultra" eventually became the standard...

, and in detection technologies such as radar

Radar

Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...

 and sonar

Sonar

Sonar is a technique that uses sound propagation to navigate, communicate with or detect other vessels...

/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

United States Coast Guard Cutter

Cutter is the term used by the United States Coast Guard for its commissioned vessels. A Cutter is or greater in length, has a permanently assigned crew, and has accommodations for the crew to live aboard...

  armed with depth charges, K-guns and Hedgehog

Hedgehog (weapon)

The Hedgehog was an anti-submarine weapon developed by the Royal Navy during World War II, that was deployed on convoy escort warships such as destroyers to supplement the depth charge. The weapon worked by firing a number of small spigot mortar bombs from spiked fittings...

.