Former CIA analyst George Allen said today that a CIA compromise almost 18 years ago on enemy troop strength in Vietnam was "the mistake of the century."

Allen, a key defense witness for CBS Inc. in retired Army general William C. Westmoreland's $120 million libel suit, called a 1967 agreement between the Central Intelligence Agency and Westmoreland's command on how many enemy troops were in Vietnam late that year "a prostitution of the intelligence process."

"I felt that my own professional integrity had been compromised by my going along with this particular estimate and that . . . the agency had sacrificed its integrity on the altar of public relations and political expediency by going along with the publication of a dishonest and misleading estimate," Allen said.

Allen, 58, who was a former deputy chief of Vietnam affairs for the CIA and is under contract to lecture on intelligence ethics at the agency, said he told CBS producer and co-defendant George Crile that Westmoreland "had the fundamental responsibility" for the "distortion of the intelligence process."

He said Westmoreland, commander of ground forces in Vietnam at the time, established a "command position" that listed enemy troop totals in the 1967 official roster at about 300,000 instead of as much as 500,000. The higher figure was proposed by the CIA and some of Westmoreland's Army intelligence experts.

Allen's firm defense of CBS came on the third anniversary of the broadcast at issue in this case. Called "The Uncounted Enemy: A Vietnam Deception," the program accused Westmoreland of being part of an alleged "conspiracy" to keep a ceiling on enemy troop-strength figures in order to maintain support for the war.

Westmoreland, who has argued that the broadcast defamed him in saying he tried to hide the larger enemy count from superiors including President Lyndon B. Johnson, has testified that his officers dropped "home militia troops" from the official enemy count because they were difficult to count and were "civilians" or "non-soldiers."

Allen said such troops should be considered dangerous in a the type of war waged in Vietnam. "The militia was organized in much the same way as our own militia had been during the Revolutionary War," he said.

Allen said the importance of the troop estimate became apparent in January 1968 when communist forces staged the Tet offensive, the series of attacks against virtually every major city and military base that became for many Americans a psychological turning point affecting their support for the war.

"This was the chickens coming home to roost," Allen said he told codefendant Samuel A. Adams, who worked for Allen at the CIA in 1967 and early 1968. "Our having gone along with the dishonest estimate had contributed to the psychological impact on the administration of the Tet offensive," Allen said.

Allen said he spoke to Crile "more candidly and forthrightly" off camera than during the two interviews he gave the team working on the disputed documentary.

"I had some feeling of guilt about my involvement . . . and was reluctant publicly to acknowledge that guilt," Allen testified.

"I was not proud of my own involvement in this," he said, speaking firmly to the jury. "I was not proud of the agency's involvement, and I just did not feel that I was prepared at that time to wash my own and the agency's dirty linen in public."

Allen said he felt that he was under similar constraints for the broadcast as those he said were imposed on him by then-CIA Director William E. Colby in 1975 when Allen testified before the House intelligence committee.

Challenged later by Westmoreland's attorney, David M. Dorsen, about his statements to that panel, Allen said Colby told him before his appearance that "we . . . don't want to put ourselves in the position of attacking the military or appearing to attack the military in order to save the agency on this issue."

"I played my role on that occasion, I regret to say, of not breaking ranks and conforming to what I now see clearly in my view was a whitewash," Allen said.