As a result of his action on Thursday, most of what was posted on the website of the National Archives and Records Administration was not entirely new. Just 53 of the 2,891 documents had never been released before. The rest had previously been released, with some information blacked out, and were made fully public on Thursday.

That means about 3,000 documents remain secret in their entirety, and about 27,000 others remain partly hidden. Under Mr. Trump’s order, those should be released in full by April 26 unless the intelligence agencies make a compelling case to withhold portions of them.

Historians who have examined many of the documents released this week said they have not found much that would drastically change their understanding of the Kennedy assassination. But these files, along with another batch made public in July, reinforce existing narratives, raise further questions or add additional details and context to story lines that were previously part of the record.

In particular, the historians said, the documents help flesh out the backdrop for the shooting that felled the young president in Dallas on Nov. 22, 1963, and the blizzard of conspiracy theories that followed.

At the center of the superpower standoff was Cuba, then as now a Communist beachhead 90 miles from the United States and an obsession for Kennedy and his brother Attorney General Robert F. Kennedy. Seared by the fiasco of the Bay of Pigs incursion and leery after the nuclear near miss of the Cuban Missile Crisis, the Kennedys spent much of their time in power exploring ways to counter and possibly topple Fidel Castro.

One document included in the July release outlined a proposal to cause food shortages in Cuba as part of Operation Mongoose, the Kennedy administration project to remove Castro from power. The plan called for introducing untraceable biological agents to destroy crops in Cuba, leading to widespread hunger that could set off a revolt against Castro.

According to the memo, Kennedy’s national security adviser, McGeorge Bundy, “said that he had no worries about any such sabotage which could clearly be made to appear as the result of local Cuban disaffection or of a natural disaster, but that we must avoid external activities such as the release of chemicals, etc., unless they could be completely covered up.” Like many of the plans, there is no evidence that it was ever executed.