the justified and legitimate organic anxiety of a nation facing a rising totalitarian threat from communism. communists were invading and annexing countries on the european continent, colonizing and overthrowing the governments in other parts of the globe, and building weapons in a bid to eliminate all global alternatives to the red communist system.
the first era of the red menace was in the 19-teens and twenties. this was a time when progressives loyal to the russian revolution were setting off bombs in u.s. cities and attempting to assasinate u.s. presidents.
the second era was spurred by the revelation that the u.s. state department had become ridden with soviet spies. this , in conjunction with overt soviet power maneuvers in the hemisphere caused americans to support greater vigilance from their leaders.
communism is a doctrine of slavery which literally reduces a person's value to the cost of their labor. the fact that most progressives believe that it is acceptable to kill to bring this system home is why most people perceived communism to be a red menace.