Costa Rica declares War on Germany
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.
Costa Rican President Federico Tinoco had his own reasons for declaring war on Germany. Tinoco had seized power in a coup and had been refused recognition by President Wilson. Accordingly to gain recognition he broke relations with Germany in Sep-1917 and followed this with a declaration of war in the following May. With the breach of relations he also interned all of the German residents - more to prevent their assisting his predecessor to regain power. This record did not help him, as the United States continued to deny recognition and even prevented Costa Rica from taking part in the Peace Conference.