Sabra & Shatila, why didn't the Americans stop the Israelis?

(The picture was taken from the site of the Institute for Palestine Studies, which also carries several  articles about Sabra & Shatila for the occasion of this unhappy anniversary).

  These days we remember that exactly 30 years ago, on 16,17 and 18 September 1982, a massacre took place in the Palestinian camps Sabra and Shatila in West-Beirut. The slaughter occurred at the end of Israeli invasion of that year with the purpose of driving the Palestinians and Syrians out of Lebanon and installing an Israel-friendly regime in Beirut.
 The Israelis started the war on 6 June. On its way north they destroyed a number of Palestinian camps in Tyrus, Sidon and south of Beirut, and began a siege of West-Beirut that lasted from July until September. Then, after much destruction and a death toll among Lebanese and Palestinians that may well have been over 10.000, the Americans stepped in. Special envoy Philip Habib negotiated a departure of the Palestinian fighters. The Palestinians ad Syrians chased away, it seemed as if Israel would have its way. Bashir Gemayel, leader of the Christian Phalangist militia, an ally of the Israelis was poised to become Lebanon's president in a few weeks time.
But on 14 September,  Gemayel was killed by a  bomb in his headquarters. The next day, in spite of the  Habib agreement Israel occupied West-Beirut. One day later it led Phalangist forces into the Sabra and Shatila camps, who - with the help of the Israeli's who provided food, and at night flares in order to illuminate the dark alleys in the camps -  in the about 60 hours that followed killed some 3000-3500 people, most elderly, women and children. That at least was the number of victims that Israeli journalist Amnon Kapeliouk, in his book 'Sabra & Shatila, Inquiry ito a Massacre' came to consider as the most plausible.        
 Many questios have been asked over the years as to why the Americans did not stop the Israelis from entering West-Beirut and did not protect the Palestinians as they promised in the agreement of Habib. However, today  Seth Anziska provides some interesting answers in The New York Times: 

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. 
Read the full article here.