On Sept. 11, 1939, the President of the United States sent a very unusual letter to a subordinate minister in the British Government:

"It is because you and I occupid similar positions in the World War (I) that I want you to know how glad I am that you are back again at the Admialty . . . What I want you and the Prime Minister to know is that I shall at all times welcome it if you want to keep in touch with me personally with anything you want me to know about. You can always send sealed letters through your pouch or my pouch."

That letter - from President Franklin Delano Roosevelt to Winston Churchill, then the Admiralty's first lord under British Prime Minister Neville Chamberlain - was to open a remarkable correspondence in modern history.

The messages, numbering about 1,800 and exchanged largely by cable outside official channels, were unsealed a few years ago and inspired a fresh look at "Roosevelt and Churchill, 1939-1941" by Joseph P. Lash, who won a Pulitzer Prize for his earlier "Eleanor and Franklin."

"After Eleanor and Franklin, where can you go? Only down, I suppose," Lash said yesterday and smiled happily.

The critical and popular success of "Eleanor and Franklin," make into an award-winning TV mini-series last year with a second series due this spring is enough to warm any historican's checkbook. Particularly a former newspaperman's.

In a sense, it all dates back to the 1930s, when Lash came to Washington as a "youth agitator."

"I led parades for relief for unemployed youth and was part of the American Youth Congress and other groups. We got hauled before the Dies Committee investigating subversive groups."

Mrs. Eleanor Roosevelt was sympathetic to the young people's causes at the time, and this was how youthful agitator Lash came to know the First Lady, an association that has enriched history and the author.

"When we were being questioned by the Dies Committee," Lash recalled, "Mrs. Roosevelt came down and sat with the newspaper people in the press section. I think she thought her presence might influence the committeemen to treat us a little more civilly. Apparently she liked some of my answers to the questions, and anyway that led to our friendship."

That friendship was close enough that Lash was invited to lunch at the White House on the New Year's Day after Pearl Harbor, when Churchill came to the United States.

He has memories of Churchill talking about bathtubs as a contrivance "foisted" upon the British by Americans, while conceding the joys of "lying back and kicking one's legs in the air - as at birth."

But the hard core of Lash's histories are not personal reminiscences but the fruits of a scholar's lonely search - Not without its own special excitments - through the documents of history.

Yesterday, former newspaperman Lash, arrayed in a magnificently shaggy scholar's beard, spoke with absolute joy of the delights of researching amid the British government's official records.

"It is a paradise for researchers," he said. "Really an historian's bonanza. Say a cable came from the British ambassador in Washington. A third secretary would summarize reactions. You story is practically written for you.

"As the dispatch went up each echelon in the foreign office, comments were added. The people on the American desk were no slouches in the use of the English language."

Lash, who practically lived at the Hyde Park library during his research or the Roosevelts, will be using both the British and American files on a new book, which moves on in time and expands to embrace the relationship between Roosevelt, Churchill and Stalin.

After his days as a youth agitator, Lash put in a stint as an Army weatherman in the early 50s became United Nations correspondent for the New York Post. That led to his first foray into history - which, he admits happily, has given him a "pleasant new life" as a historian.

He had been covering Dag Hammarskjold, as secretary general of the United Nations, and Harper's megazine published a Lash article on "The Man on the 38th Floor." This led to a book profile of Hammarskjold. This was before the publication of Hammarskjold's private diary jottings in "Markings," which revealed the troubled, agonizing man under the austere Scandinavian surface of the public man.

Later came the books on the Roosevelts. Lash can find little to fault in the telvision series based on his book "Eleanor and Franklin."

"I kept veto rights over the scripts because of a sense of responsibility to the Roosevelts. But I find it very difficult to exercise the veto right," he concedes happily.

Jane Alexander's interpretation of Elanor Roosevelt brings only praise from Eleanor's biographer.

"The voice and expression is not mimicry, yet it is so unfailingly true," he marvels.

As for a newspaperman turning into a historian's historian, Lash doesn't see any contradiction of disciplines.

"I don't want to upset any scholars," he said with a mischievous chuckle. "But a good newspaperman who has some perspective and pays attention to his sources . . . and an eye for telling detail . . . can do a good history.

In "Roosevelt and Churchill, 1939-41," Lash uses telling detail to relate his readable drama of history and two "enormously egocentric men whose affection altered the course of history."

There is, for instance, the two young government officials who met briefly in 1918 and then 23 years later for Atlantic Charter talks at Argentia, the U.S. naval base in Newfoundland. FDR was noticeably hurt that Churchill had forgotten the first meeting.

"Yet, the irony is that Roosevelt, in his long letters to Eleanor in the form of a diary, never considered Churchill important enough to mention," Lash added as a footnote in the interview yesterday.

Other Lash vignettes of two leaders and history:

Both Churchill and Roosevelt knew the importance of having information, that information "not only is essential to wise judgment but conferred authority on the possessor." Churchill kept himself well-informed while out of government in '30s.

The final exchange between the two via the U.S. Embassy in London not only was unorthodox but raised a sticky matter of protocol. The American ambassador was when Joseph P. Kennedy, who favored appeasement of Hitler. But Churchill decided it was best not to bypass Kennedy and court his ire.

Churchill couldn't resist a turn of phrase even if it was ompilitic. To a colleague who advocated continued caution with Americans even after the United States was in the war, the British prime minister replied:

"Oh! That is the way we talked to her while we were wooing her; now that she is in the harem, we talk to her differently."

In his book, carefully phrased without stated conclusions, Lash raises the question of the parallels between this country's road to Pearl Harbor and to Vietnam. Is a strong executive always dangerous and foreign intervention for any reason always disastrous? he asks.

Yesterday Lash talked about the differences in Roosevelt's emergency powers and the unsurpations of presidential power in the road to Vietnam.

"In 1940-41, Roosevelt concluded that America's vital interests were involved in the war in Europe. I think that history has justified Roosevelt that his calculation was correct. In the case of Vietnam, no one ever demonstrated to the American people that our vital interests were involved. It was a peripheral war that the leaders tried to invest with some kind of anti-Communist campaign."