Complex sentence: The United States did form alliances with non Algonquian Indians (the Dakota, Sahaptin, Salish, and Uto) before 1860, which the present day Dakota, Sahaptin, Salish, and Uto now regret.
Split into simpler sentences: The United States did form alliances with non Algonquian Indians (the Dakota, Sahaptin, Salish, and Uto) before 1860. An action the present day Dakota, Sahaptin, Salish, and Uto now regret.