LONDON — Thirty-year-old documents newly released by the British government reveal just how severely America’s decision to invade the Caribbean island of Grenada in 1983 tested the warm ties between Prime Minister Margaret Thatcher and President Ronald Reagan.

While the two leaders had a strong and affectionate personal rapport, the British official papers reveal how little warning Mrs. Thatcher was given about the pending military invasion, a move that left the British irritated, bewildered and disappointed. They also show how Mr. Reagan justified the secrecy as a way to prevent leaks, and how the British later concluded that the invasion had in fact been planned long in advance. At one point during tense written exchanges, both leaders claimed, in defense of their opposing approaches to the unrest in Grenada, that lives were at stake....