ON the night of Sept. 16, 1982, the Israeli military allowed a right-wing Lebanese militia to enter two Palestinian refugee camps in Beirut. In the ensuing three-day rampage, the militia, linked to the Maronite Christian Phalange Party, raped, killed and dismembered at least 800 civilians, while Israeli flares illuminated the camps’ narrow and darkened alleyways. Nearly all of the dead were women, children and elderly men.

Thirty years later, the massacre at the Sabra and Shatila camps is remembered as a notorious chapter in modern Middle Eastern history, clouding the tortured relationships among Israel, the United States, Lebanon and the Palestinians. In 1983, an Israeli investigative commission concluded that Israeli leaders were “indirectly responsible” for the killings and that Ariel Sharon, then the defense minister and later prime minister, bore “personal responsibility” for failing to prevent them.

While Israel’s role in the massacre has been closely examined, America’s actions have never been fully understood. This summer, at the Israel State Archives, I found recently declassified documents that chronicle key conversations between American and Israeli officials before and during the 1982 massacre. The verbatim transcripts reveal that the Israelis misled American diplomats about events in Beirut and bullied them into accepting the spurious claim that thousands of “terrorists” were in the camps. Most troubling, when the United States was in a position to exert strong diplomatic pressure on Israel that could have ended the atrocities, it failed to do so. As a result, Phalange militiamen were able to murder Palestinian civilians, whom America had pledged to protect just weeks earlier.

Israel’s involvement in the Lebanese civil war began in June 1982, when it invaded its northern neighbor. Its goal was to root out the Palestine Liberation Organization, which had set up a state within a state, and to transform Lebanon into a Christian-ruled ally. The Israel Defense Forces soon besieged P.L.O.-controlled areas in the western part of Beirut. Intense Israeli bombardments led to heavy civilian casualties and tested even President Ronald Reagan, who initially backed Israel. In mid-August, as America was negotiating the P.L.O.’s withdrawal from Lebanon, Reagan told Prime Minister Menachem Begin that the bombings “had to stop or our entire future relationship was endangered,” Reagan wrote in his diaries.

The United States agreed to deploy Marines to Lebanon as part of a multinational force to supervise the P.L.O.’s departure, and by Sept. 1, thousands of its fighters — including Yasir Arafat — had left Beirut for various Arab countries. After America negotiated a cease-fire that included written guarantees to protect the Palestinian civilians remaining in the camps from vengeful Lebanese Christians, the Marines departed Beirut, on Sept. 10.

Israel hoped that Lebanon’s newly elected president, Bashir Gemayel, a Maronite, would support an Israeli-Christian alliance. But on Sept. 14, Gemayel was assassinated. Israel reacted by violating the cease-fire agreement. It quickly occupied West Beirut — ostensibly to prevent militia attacks against the Palestinian civilians. “The main order of the day is to keep the peace,” Begin told the American envoy to the Middle East, Morris Draper, on Sept. 15. “Otherwise, there could be pogroms.”