Haiti Declares War on Germany
A month after the United States declared war Haitian President Sudre Dartiguenave asked the Haitian Congress for a declaration of war because of the destruction by a German submarine of a French steamer which had among its crew and passengers Haitian citizens. Congress refused but a few days later adopted a resolution condemning unrestricted submarine warfare and empowering the president to break diplomatic relations with Germany if that country refused reparations and guarantees for the future. Haiti eventually declared war, one of the last to do so.
Of course, these levels of 'involvement' differed hugely. Some countries mobilized millions of troops and fought hard for over four years, some were used as reservoirs of goods and manpower by their colonial rulers, while others simply declared war late on and contributed only moral support. Many were drawn in by colonial links: when Britain, France and Germany declared war they were also committing their empires, automatically involving most of Africa, India and Australasia, while the entry of the US in 1917 prompted much of central America to follow.