(1) His gut feeling - that Communists would stop at nothing to infiltrate the US government, and that given the law of averages they had to have been somewhat successful - was right.
(2) He actually did not have concrete proof of many of his allegations. He was right about some of the people he accused and wrong about others.
Had he been as cautious in his research as he was accurate in his assumptions, he would have been far more effective. His botched invstigation into the Army at Fort Monmouth destroyed his reputation, and he kind of asked for it.
Who was he wrong about?