WASHINGTON, Nov. 28 — In July 1969, as the world was spellbound by the Apollo 11 mission to the moon, President Richard M. Nixon and his close advisers were quietly fretting about a nuclear arms race in the Middle East. Their main worry was not a potential enemy of the United States, but one of America’s closest friends.

“The Israelis, who are one of the few peoples whose survival is genuinely threatened, are probably more likely than almost any other country to actually use their nuclear weapons,” Henry A. Kissinger, the national security adviser, warned Mr. Nixon in a memorandum dated July 19, 1969 — part of a newly released trove of documents.

Israel’s nuclear arms program, which Israel has never officially conceded exists, was believed to have begun at least several years before, but it was causing special problems for the young Nixon administration. For one thing, the president was preparing for a visit by its prime minister, Golda Meir, who was also in her first year in office and whose toughness was already legendary.

Should Washington insist that Israel rein in its development of nuclear weapons? What would the United States do if Israel refused? Perhaps the solution lay in deliberate ambiguity, or simply pretending that America did not know what Israel was up to. These were some of the options that Mr. Kissinger laid out for Mr. Nixon on that day before men first walked on the moon.