By the early 1960s doubts emerged about the ability of Bomber Command to pierce the defences of the Soviet Union. The shooting down of a U-2 spyplane in 1960 confirmed that the Soviet Union did have surface-to-air missiles capable of reaching the heights at which bombers operated. Since the Second World War the philosophy of bombing had involved going higher and faster. With the supersession of high and fast tactics, ultra-low-level attack was substituted. Bomber Command aircraft had not been designed for that kind of attack, and airframe fatigue increased. All Valiants were grounded in October 1964 and permanently withdrawn from service in January 1965. Low-level operations also reduced the lifespan of the Victors and Vulcans.