What historical examples show Mark Twain's opposition to imperialism?

Mark Twain was an imperialist until 1899, when he abruptly changed his mind over the question of American policy in the Philippines. His reasoning, in a nutshell, was that if other countries were to emulate America's success, they would only do so by achieving freedom and self-determination, as America had done. In October of 1900, Twain wrote in the New York Herald:

It should, it seems to me, be our pleasure and duty to make those people [in the Philippines] free, and let them deal with their own domestic questions in their own way. And so I am an anti-imperialist. I am opposed to having the eagle put its talons on any other land.

This article is gathered together with all Twain's other published writing on the subject in Mark Twain's Weapons of Satire: Anti-Imperialist Writings on the Philippine-American War edited by Jim Zwick (Syracuse University Press, 1992). This is the best collection of primary sources available in a single volume. Zwick was the leading scholar on Twain's relationship to imperialism, and a good overview of the subject is his essay "Mark Twain and Imperialism" in A Historical Guide to Mark Twain, edited by Shelley Fisher Fishkin (Oxford University Press, 2002). The internet site, twainweb.net, which is a very good general resource for Twain-related material, put up a special memorial page for Zwick following his death in 2008. This page is useful because it includes a bibliography of all Zwick's academic writing, including many publications on Twain and imperialism (link below).

A well-written and accessible recent account of Twain's anti-imperialist activities during the last decade of his life is contained in Stephen Kinzer's book, The True Flag: Theodore Roosevelt, Mark Twain, and the Birth of the American Empire (Henry Holt, 2017). This links Twain's views to contemporary debates about American imperialism. Finally, for a very basic overview, and a few excerpts from Twain's writing, the historywiz website (link below) is a good starting point.