How high a military aircraft can fly partly depends on what an airplane is and whether it's 'flying'. By some definitions, the SR-71 "Blackbird" was the fastest manned aircraft in the world and claimed the record for the highest flight. But both the X-15 and MiG E-266M dispute this claim. Interestingly, all three of these military records were set during the competition of the Cold War in the 1960s and 1970s and haven't been broken since. It seems that flying higher and higher isn't the main focus of military aviation.