As the growing sophistication of air defenses pushed aircraft to the limits of aerodynamic flight and the limitations of orbiting reconnaissance satellites became apparent in the 1960s, the need for a new class of reconnaissance capability emerged above most aircraft and threats but below the low orbiting satellite and with a longer dwell time than either a pseudosatellite, or "pseudolite." The U-2 had to fly in the "coffin corner" of its flight envelope, one knot from stalling, one knot from overstressing the airframe, and one SA-2 from an international incident. The satellites could glimpse but not watch, and soon those watched knew when to draw the curtains closed. The U.S. Defense Department began pursuing a pseudolite capability based on unmanned aircraft during the latter days of the Vietnam War. They were referred to as high-altitude long-endurance (HALE, or later just HAE) UAVs, where "high" was understood to be above 50,000 ft and "endurance" over 24 h.