From Cold Warfare: A Compact History, pages 38 to 47:

The years 1946 and 1947 were ones of increasing friction with the Soviet Union. They would also determine the fate of US intelligence until the present time. We can get a good view of attitudes in the American government at the time from a 1946 telegram written by George F. Kennan, the Charge d' Affairs of the American embassy in Moscow. The 8,000 word "long telegram" was drafted in response to a query from Washington. In part Kennan states:

[The] USSR still lives in [an] antagonistic "capitalist encirclement" with which in the long run there can be no permanent peaceful coexistence… We have here a political force committed fanatically to the belief that with the US there can be no permanent accommodation, that it is desirable and necessary that the internal harmony of our society be disrupted, our traditional way of life destroyed, the international authority of our state broken, if Soviet power is to be secure.

In July of 1947, under the pseudonym "X" Kennan outlined what he felt were the irreconcilable differences in US and Soviet society and he made some recommendations for US policy makers. In an article in Foreign Affairs magazine he noted the importance of ideology as a cause of Soviet conduct. Kennan recommended:

...the United States entering with reasonable confidence upon a policy of firm containment, designed to confront the Russians with unalterable counter-force at every point they show signs of encroaching upon the interests of the peaceful and stable world...

...the United States has in its power to increase enormously the strains under which Soviet policy must operate, to force upon the Kremlin a far greater degree of moderation and circumspection than it has had to observe in recent years, and in this way to promote tendencies which must eventually find their outlet in either the break-up or the gradual mellowing of Soviet power.

In February of 1948, the year after the CIA had been created, and while Kennan was assigned to the position of director of the Policy Planning Staff in the US department of State, he wrote the then top secret memorandum PPS #23 which was entitled "Review of Current Trends: US Foreign Policy." In part he advocated that the newly created CIA be given covert action capability, not just intelligence gathering abilities. Kennan said,

It cannot be too often reiterated that this government does not possess the weapons, which would enable it to meet head-on the threat to national independence presented by the communist elements in foreign countries.

Years latter in his Memoirs Kennan explained:

The thought was entirely sound. It was not a question of establishing what became referred to as "a department of dirty tricks." It was a question of creating a facility designed to give greater flexibility to the operations of government, now involved in a global Cold War, whose traditional arrangements for the appropriation and use of public funds was wholly unsuited to such a role in world affairs.

Under pressure from Kennan, the capabilities of the CIA were to be enhanced to include the ability to undertake covert paramilitary operations. On June 18, 1948, the centennial year of the Communist Manifesto, the National Security Council (NSC) authorized the CIA to create the "Office of Special Projects" to undertake covert actions. The then Top Secret National Security Directive 10/2 read as follows:

The National Security Council, taking cognizance of the vicious covert activities of the USSR, its satellite countries and Communist groups to discredit and defeat the aims and activities of the United States and other western powers, has determined that, in the interest of world peace and US national security, the overt foreign activities of the US government must be supplemented by covert operations.

NSC directive 10/2 gave the president and the NSC permission to fight the Cold War using the following methods:

Propaganda; economic warfare; preventative direct action, including sabotage, anti-sabotage, demolition, and evacuation measures; subversion against hostile states, including assistance to underground resistance movements, guerrilla and refugee assistance groups, and support of indigenous anti-Communist elements in threatened countries of the free world. Such operations shall not include armed conflict by recognized [US] armed forces.

In his autobiography Honorable Men former CIA director William Colby write of this move:

With the rising intensity of the Cold War and the clear use by the Communist world of infiltration, subversion, and guerrilla techniques - in Greece, in Western Europe, and in the Philippines - an increasing number of voices argued that the United States had to have not only the intelligence collecting and analysis capability to report on the Cold War, but also the political and paramilitary capability with which to fight.