Canada made a serious military commitment to NATO by keeping a full army brigade and air squadrons in Europe, with Canadian ships and aircraft tracking Soviet submarines and participating in military exercises. By joining NATO, Canada had to adapt its defense policy to its allies. As long-range bombers capable of carrying warheads over long distances were developed, North America became vulnerable to attack, so the US built three lines of radar stations across Canada to detect any surprise attacks and allow time for a counterattack.