Well, it’s no good talking about Canada during the American Revolution; Canada had no separate national identity at the time. Indeed, to the American’s way of thinking, the French who then constituted most of Canada would happily join in against the hated British, who had only conquered them 13 years earlier. They did not allow for how the British had carefully wooed the French (in 1776, a Canadian Roman Catholic had full civil rights, which English Catholics didn’t get until 1829), or for how the Canadians did not trust the more radically Protestant Americans.
Vietnam (backed by the Soviet Union) battled Cambodia (backed by China) during the 1970s.
Not sure about your main point though. Economists tend to be pretty skeptical about cost/benefit analysis as applied to war given the devastation they cause. Admittedly, your argument wasn’t strictly pecuniary.
Am I to take from that you don’t know what or where the proxy wars were?
But I think Friedman’s corollary that no two countries with a McDonalds have gone to war.
Let’s say I don’t see how any of them could be plausibly conceived of as counting as “wars between democracies”. An example naming all the countries involved in a particular case and which of those countries are the two “democracies” the war in question is to be taken as being “between” might make it clearer.
There are at least 2 examples of democracies engaging in war: the first is better known - Finland and the United Kingdom during WWII, the second is a little more obscure - the Cod Wars between the UK and Iceland (no really - Cod Wars - Wikipedia).