The only case in which this would be a valid argument, is to protect against a small military coup. Nazism, Fascism and Communism have all had huge popular support and most of the time it's been a people's movement, so an armed populace won't do anything against that, as the fascist/nazi/commie supporters will also have the same guns. The most probable outcome in such a case is a long civil war. And we all know how every government act in times of civil war, don't we?
An armed population will have zero chance against a foreign military invasion, see Iraq/Afghanistan. Their only chance is, like in Iraq and Afghanistan, to win a war of attrition.
Indeed.
Yes, we've never seen an extremist popular movement end up in a ruthless dictatorship... That's never happened.
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