Communists can place no trust in bourgeois legality

History of Soviet espionage in the United States
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In almost all the countries of Europe and America, the class struggle is entering the phase of civil war. In these conditions, Communists can place no trust in bourgeois legality. They must everywhere build up a parallel illegal organisation, which, at the decisive moment, will be in a position to help the Party fulfil its duty to the revolution., Comrade V.I.Lenin, July 1920.
Secret apparatus
In the 1930s CPUSA membership became largely native-born, and more educated people joined, including many scientific and technically trained professionals. American Communists considered the 'capitalist' corporations which employed them as morally illegitimate institutions. When Soviet intelligence officers approached and asked that the scientific secrets of these corporations be shared with the Soviet Union, few had moral objections.
Soviet recruitment of sources within American intelligence agencies, particularly within the Office of Strategic Services (OSS), the predecessor to the Central Intelligence Agency was impressive. The highest ranking recruit was Duncan Lee, counsel to General William Donovan, OSS head. Lee, however, was extremely cautious and less productive than other OPGU sources, like Maurice Halperin, or Donald Wheeler, in the OSS Research and Analysis division. At least fifteen Soviet agents penetrated the OSS, with the actual number more likely around twenty.
The Soviets also developed about twenty sources within the U.S. State Department and other wartime foreign relations agencies. The two most senior Soviet operatives had been active in the 1930s, Alger Hiss and Laurence Duggan. A number of other Soviet infiltrators connections to American diplomacy have only been identified by code-names in the Venona project materials. Some of these identities have not been determined. Some most likely continued to operate in the post-war period. American counter-intelligence officials spent decades interviewing and examining the backgrounds of hundreds of American diplomatic personnel attempting to attach an identity to the code-name of many of these known operatives.
Infiltration
Infiltrating the United Nations organization became a priority in the wake of the disbanding of the Comintern, the death of Golos which led to the ultimate breakdown in security, and the end of the War. Hiss was influential in the employment of 494 persons by the United Nations on its initial staff.
Gradually it became apparent that the objectives of World War II for which the United States and others made tremendous sacrifices were not fully realized, and there remained in the world a force presenting even greater dangers to world peace than the Nazi militarists and Japanese warlords. Consequently, the United States made the decision in the Spring of 1947 to assist Greece and Turkey with a view to protecting their sovereignties, which were threatened by direct or instigated activities of the Soviet Union.

President Truman's Executive Order 9835 of 22 March 1947 tightened protections against subversive infiltration of the US Government, defining disloyalty as membership on a list of subversive organizations maintained by the Attorney General.

Truman’s denunciations of the charges against Alger Hiss, Harry Dexter White, and others, all of whom appear under covernames in decrypted messages translated before Truman left office, suggest that Truman was never briefed on the Venona program, or if he was briefed, did not grasp its significance. Truman insisted Republicans trumped up the loyalty issue, and that wartime espionage had been insignificant and well contained by counteritelligence agencies.

Senator Daniel Patrick Moynihan, Chairman of the Moynihan Commission on Government Secrecy wrote in 1997, "President Truman was almost willfully obtuse as regards American Communism."