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You do realize that the US was in Lebanon as part of an multinational peacekeeping force? Their mission? To oversee the safe evacuation of PLO guerrillas. But im sure you knew that...
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"A U.S. contingent entered Lebanon in July 1982 to oversee the departure of the Palestine Liberation Organization, which Israel had invaded to displace. That American detachment left in September 1982, but U.S. forces returned later that month when violence resumed. "
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They were there before as part of the PLO evacuation. This was a different mission that they got bombed for.