I have been trying to accumulate a large archive of anti-war writings from a multitude of individuals and organizations so I wanted to post the ones I have for others and see if anyone else has more to give.
W.G. Sumner - The Conquest of the United States by Spain
Edwin L. Godkin - The Eclipse of Liberalism
Roderick T. Long - They Saw It Coming: The 19th-Century Libertarian Critique of Fascism
Thomas E. Woods Jr. - The Anti-Imperialist League and the Battle Against Empire
If you have any that are not covered here, please add to them.