In the early years of America's entry in World War II, the U.S. military set up early warning radar equipment along the coastlines over concerns regarding the country's defense against enemy bombers. After the surprise attack on Pearl Harbor, officials were worried that the enemy might target the Panama Canal in particular due to its importance in connecting military and shipping routes of the Atlantic and the Pacific. Since the western entrance to the Canal Zone did not have any land areas nearby that could be used as radar sites, the entrance was guarded by picket ships carrying radars. By 1942, thoughts shifted from passive air defense to Allied offensives, and the military leaders in the Air Force stressed the need for portable long-range aircraft detectors during their battles in the Pacific. However, American radar equipment at the time had a frequency ceiling of 200 megacycles, which made them not only vulnerable to surprise raids by low-flying aircraft but also extremely bulky and difficult to scale down.