Twain was very active with the American Congo Reform Association
from 1904 to 1906.  He was one of its vice presidents (along with
Booker T. Washington, Henry Van Dyke, and others) and made several
lobbying trips to Washington on behalf of the organization.  After
Harper & Brothers rejected his essay King Leopold's Soliloquy in
April 1905, he had to obtain their permission before allowing the
American Congo Reform Association to publish it as a pamphlet.

I think this episode deserves more attention than it has received in
Twain biography and criticism.  It happened the month after Harpers
rejected "The War Prayer" and Twain wrote in his notebook that "In
America -- as elsewhere -- free speech is confined to the dead."
While he was waiting for Harpers' permission to give King Leopold's
Soliloquy to the Congo Reform Association he technically broke his
exclusive contract with them by publishing an essay on "Christian
Citizenship" anonymously in the Sept. 2, 1905, issue of _Colliers
Weekly_ -- then wrote to Harpers saying that he'd published something
elsewhere and challenging them to find it.  A few months later he was
working full-time on his autobiography which he planned for
publication after he died.  After that he continued to give
interviews and speeches about imperialism in the Philippines and the
Congo, and about the Russian Revolution, but he did not submit any
other anti-imperialist essays for publication by Harper & Brothers.
Although the Anti-Imperialist League, which Twain was also serving as
a vice president, published two pamphlets about the Moro massacre in
the Philippines in March of 1906, Twain put his own autobiographical
dictations about it aside as "not usable yet" and he later removed
the anti-imperialist chapter in Captain Stormfield's Visit to Heaven
before giving the remaining "Extract" to Harpers.  To what extent did
his experiences with Harper & Brothers from March through September
of 1905 influence his perceptions of what he could publish and
decisions about what to write during his remaining years?

A facsimile edition of King Leopold's Soliloquy is included in the
Oxford Mark Twain volume, _Following the Equator and Anti-Imperialist
Essays_ (Oxford U Press, 1996).

There is a little about this involvement online in chapter 234 of
Albert Bigelow Paine's _Mark Twain: A Biography_
http://marktwain.miningco.com/library/biography/bl_paine_bio_ch234.htm

Among the best articles about it are:

Giddings, Robert.  "Mark Twain and King Leopold of the Belgians."  In
Robert Giddings, ed., _Mark Twain: A Sumptuous Variety_ (Totowa, NJ:
Barnes & Noble Books, 1985): 199-221.

Hawkins, Hunt.  "Mark Twain's Involvement with the Congo Reform
Movement: 'A Fury of Generous Indignation.'" _New England Quarterly_
51:2 (June 1978): 147-175.

Royot, Daniel.  "The Fantastic Record of a Maniac: King Leopold's
Soliloquy Revisited."  In Serge Ricard, ed., _An American Empire:
Expansionist Cultures and Policies, 1881-1917_ (Aix-en-Provence:
Publications de l'Universite de Provence, 1990), 237-243.

Jim Zwick