The United States desperately wanted to prevent Israeli troops from invading Beirut to capture or kill the PLO leadership hiding there. While expressing support for Israel's security, the administration was also alarmed by the anger toward Israel and the United States in the Arab world provoked by the Israeli invasion. In an effort to end the fighting and convince Israel to withdraw, Secretary of State George Shultz negotiated a deal to allow the PLO to be safely evacuated out of the country and an international peacekeeping force including American troops to deploy in Beirut. Click here to read the transcript of Shultz's press conference.
Source: U.S. State Department