Might also mention Pierre Trudeau as an effective middle power leader.
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I didn't think he was particularly good at it, which is why I didn't list him with Pearson (who was excellent), and St. Laurent (who was sublime).
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Trudeau could, and did, irritate several of our allies, repeatedly - and his relationship with Nixon was the stuff of legend ("I've been called worse things by better people.")
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But Gerald Ford liked Trudeau, which is why Canada was included in the G7.  Being included in the G7 is about as influential as any middle power can ever hope to be.  Canada has been at that table for 40 years.
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So, in that way, Pierre Trudeau was ultimately quite successful.