As soon as World War II ended, the United States and the Soviet Union began a struggle for supremacy. It was against the backdrop of the Cold War that the threat of internal subversion and external attack began to preoccupy Congress. On February 9, 1950, Senator Joseph McCarthy gave a speech in Wheeling, West Virginia, where he claimed to have in his possession information proving that more than 200 employees in the State Department were Communist Party members. This began a campaign during which McCarthy continued to make accusations about communist activity within the executive branch. His 1954 probe of the U.S. Army, televised nationally, led to his downfall. The featured primary source document, a telegram from Senator McCarthy to President Truman, was sent to the President on February 11, 1950. This lesson also includes Truman's reply (probably not sent). The lesson relates to the interaction between the executive and legislative branches dealing with the nation's security and acts that may be determined to be treason (Article III, Section 3, Paragraph 1). It correlates to the National History Standards and to the National Standards for Civics and Government. The lesson provides historical background (four resources); and suggests diverse teaching activities for classroom implementation, including document analysis, research and class discussion, compare and contrast, and extension activities. (BT)