> Has there ever been a populist movement that wasn't nationalist?
Sure. American populism. It was against protectionism, against war and against imperialism. William Jennings Bryant resigned as Wilson's Secretary of State in protest against him breaking his campaign promise and bringing us into WWI. And spent the next few years campaigning furiously for ratification of the Kellogg Byrant treaties and the League of Nations.
WWI was of course the war that occasioned Randolph Bourne's "War is the Health of the State" and brought us Freedom cabbage. To campaign prominently against demonizing Germans then, even in the days just after the Lusitania was sunk, is as anti-nationalist as you can get.
Michael