AIPAC was founded in two stages. The first was in 1951, as a lobbying arm of the American Zionist Council. It was essentially a one-man operation run by Isaiah (Si) Kenen to strengthen the Israel/American relationship. He immediately went to work on getting funding from Congress for resettling Jewish refugees in Israel. He had great relationships with many members of Congress - but the State Department was notably hostile to Israel.
But the AZC lobbying arm had nothing to do with that, and it wasn't "founded to justify this massacre." That is absurd. It is not a hasbara organization.
In 1954, it was decided to spin off the lobbying group as its own separate organization with its own funding, supported by a larger base of Zionist organizations, as the battles in 1953 were time consuming (Congress reduced its funding of Israel in 1953 by about 25%.) Kenan headed the American Zionist Committee for Public Affairs, registered in March 1954.
That group was renamed the American Israel Public Affairs Committee in 1959.
All of this can be read in Kenen's 1981 book, "Israel's Defense Line: Her Friends and Foes in Washington."
As usual, Israel's enemies rely on lies - even (especially) to fundraise. Because they know that very few people will bother to research the truth.
