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.