Well you Have this Wrong, as before the War most International Commentators Were Expecting fast and Swift Peru-Bolivian Victory, and the USA was the one supporting, and actively intervening, in Favor of Peru, getting Trade, and Weapons, Embargoes Against Chile, as
"The press in the United States was also almost unanimous in predicting the sound defeat of Chile"(Mellington, Herbert (1948). American Diplomacy and the War of the Pacific. Colunbia University Press. p. 31) .
This is because Peruvian Forces come from a fairly recent process of modernization (hopeless badly done but this is not know before the war), have the most powerful Navy in the South Pacific, and the Chilean Army was in his historical low,because there was a economic crisis in the country, again because, in part, for the Bolivian kerfuffle. Chile never bought Mauser Chilean Forces Bought Gras y Comblain II, rifles, that use the same munition. What Chile did buy was Krupp Artillery.
Peru-Bolivia have all the Support the USA could muster with Chile having neither Help or Support from any European Power, but also without trade, and weapons embargoes from Europe, (except England that stop the sell to both side of warships)
Here read it in the Wikipedia,
https://en.wikipedia.org/wiki/War_of_the_Pacific#cite_ref-209
If Anything the Chilean Victory in la Guerra del Pacifico, put stop to the USA intervention in South America, as the Chilean action doing the same again in the Panama Crisis of 1885