"Canada was included in the G-7 as the USA wanted another counterpoint to the Europeans who on a voting basis would always be able to set the agenda."
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I don't disagree with this.  But the point remains that Gerry Ford was an amiable guy, and he liked Trudeau enough to include Canada.
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Ford was chosen by Nixon, but he was also one of the few choices Nixon really had.
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When Agnew had to resign, Nixon was already deep into Watergate.  Gerry Ford was chosen not because he was particularly close to Nixon, or because he was Nixon's special favorite, but because he was (with good reason) a senior legislator who everybody liked.  He had the respect of both parties, and lots of friends on both sides of the aisle.
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So the choice was made.  Ford certainly did not share Nixon's visceral hated of Trudeau.  In fact, part of why Ford was chosen was because he didn't have a visceral hatred for anybody.  He was a classy, decent guy, when America desperately needed a classy, decent guy.