In April 1981, the Israeli Air Force attacked Syrian forces in Lebanon to prevent them from seizing the strategic Sannin ridge. Syria responded by deploying surface-to-air missiles into the Biqa‘ Valley, threatening Israel’s ability to monitor PLO forces in Lebanon. To avert war, Reagan sent emissary Philip Habib to the Middle East, but he failed to persuade the Syrians to withdraw the missiles. When fighting escalated between Israel and the PLO that July, the Reagan administration feared that Israel would invade Lebanon. Ultimately, Habib managed to negotiate a de facto ceasefire between Israel and the PLO.
The ceasefire, however, merely postponed a larger crisis. The Lebanese remained at odds, Syria refused to withdraw its missiles, and Israel chafed under the restrictions of the ceasefire, which allowed the PLO to strengthen itself and did not prevent terrorist attacks from the West Bank and Gaza Strip or against Israeli and Jewish targets in Europe. In London on June 3, 1982, Palestinian assailants shot Shlomo Argov, Israel’s ambassador to the United Kingdom. The Israel Defense Forces (IDF) invaded Lebanon on June 6.