It was too fast for surface-to-air missiles, because by the time they got to it's altitude and caught up, they would have lost most of their kinetic energy. Of

It was too fast for surface-to-air missiles, because by the time they got to it's altitude and caught up, they would have lost most of their kinetic energy. Of course, modern SAMs would be able to hit it. The Soviet Union at the time had the R-40 series of air-to-air missiles, that could got Mach 4.5 at very high altitudes if launched by a fast enough, high altitude aircraft, which is much higher than the Mach 3.4 the SR-71 could hope to achieve. The reason it could go that fast is because it didn't have to expend its kinetic energy to climb much, and because it already started up at Mach 3.