Canada became involved because it was not yet a fully independent country. It was an integral part of Great Britain back then (there was no Canadian citizenship, we were all British Subjects, and the British Parliament could override ANY AND ALL Canadian legislation). In fact, until January 1, 1947, Canada was not completely independent (no country can possibly be independent, when it does not have its own citizenship).
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I always thought Canada gained complete independence in 1931 (Statute of Westminster).