Did Communism Threaten America's Internal Security After World War II?

In my opinion, I would have to agree with John Earl Haynes and Harvey Klehr. Communism did threaten america's internal security after world war II, hence the vietnam, and the cold war. McCarthyism, the intense anti-communist suspicion, happened in the U.S. during the Second Red Scare, which coincided with increased fears about communist influences on American institutions and espionage by Soviet agents. Thousands of americans were being accused of being Communists or communist sympathizers and became the subject of aggressive investigations and questioning before government or private-industry panels, committees and agencies. The primary targets were government employees, entertainment industry, educators, and union activists.
There were several attempts to introduce legislation or apply existing laws to help to protect the United States from the perceived threat of Communist subversion, such as the Smith Act, which was the first sedition law since 1798. Under the Smith Act, hundreds of Communists were prosecuted and eleven leaders of the Communist party were charged and convicted under the Smith Act. The Immigration and Nationality Act was also passed which allowed the government to deport immigrants or naturalized citizens engaged in subversive activities and also to bar suspected subversives from entering the country. Last but not least, the Communist Control of 1954 was passed, also known as an extension of the Internal Security Act of 1950, sought to outlaw the Communist Party by declaring that the party, as well as "Communist-Infiltrated Organizations" were "not entitled to any of the rights, privileges, and immunities attendant upon legal bodies".
So all in all I believe that if America had many attempts to overthrow communism that must mean that it obviously didn't benefit the internal security after world war II, it just threatened it.

Was dropping the bombs justified?

First, the bombs weren't acceptable by no means because some other action could've been taken to avoid the conflict, but the japanese did deserve it because they started it with the U.S. at Pearl Harbor. Secondly, however, some critics accused the United States of racist motives because the bombs were dropped only on nonwhite people. Since Japanese were already on the verge of collapse by 1945, the second bomb on Nagasaki was unnecessary to bring the war to a conclusion. The atomic attacks on Hiroshima and Nagasaki weren't the last shots of WWII, but the first salvos in the Cold War. Although, President Truman did infact ignore Tokyo's attempts to negotiate a surrender in the summer of 1945 and rejected all alternatives to dropping the bomb because he wanted to intimidate and isolate the Soviet Union. The bomb wasn't just another weapon, but the major instrument of destruction that would deliver victory. Thirdly, the Japanese clung to many unacceptable conditions like: protection for their imperial system of government, the right to disarm and repatriate their own troops, no military occupation of the home islands, no international trials of alleged war criminals, and possible retention of some of their conquered territories.