What made the U-2 Spy Planes initially very effective?

High-altitude U-2 spy planes began making reconnaissance flights over the USSR in 1956, giving the U.S. its first detailed look at Soviet military facilities. Did you know? U-2 pilot Francis Gary Powers carried a tiny needle filled with poison so that he could take his own life if he faced capture.

Explanation:

The U-2 Spy Planes were very effective initially, because of their ability to fly at high-altitude (~70,000 feet into the sky), as well as the ability to fly almost to a mach. Compared to many of the military hardware of the day, it was almost impossible to find them on radar, and even when it was possible, it was nigh impossible to intercept them in time.

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