About 15,000 feet. But, they rarely operated that high. Above 10-12,000 feet oxygen content of the thin air is so much less than what humans are accustomed to that flying above 10,000 feet subjects...
As far the USA is concerned; the Cold War, (the Berlin Blockade), the Korean War, the U-2 incident, the Bay of Pigs incident, the Cuban Missile Crisis, the Indochina War (Vietnam, Cambodia, Laos),...