The First World War It took place between July 28, 1914 and November 11, 1918. It involved all the great powers of the world that lined up on two opposing sides:on the one hand, the Allies of the Triple Entente, and, on the other, the Central Powers of the Triple Alliance. They declared war on each other and Andorra, caught up in that conflict, did not want to be outdone and in August 1914 declared war on Kaiser Wilhelm II , Emperor of Germany.
Logically, their participation was merely testimonial, at that time the Andorran army was made up of ten soldiers, part-time , which met a couple of times a year on the occasion of the replacement of its members. So, the war passed without pain or glory. With the end of the war, came the Paris Peace Conference (1919) and the corresponding peace treaties between the different countries that participated in the war.
As in practice Andorra had not intervened in the dispute, it was not invited to the Peace Conference either and, therefore, did not sign peace with Germany. That error was corrected with the signing of a peace treaty on September 25, 1939 (24 days after the start of World War II). Therefore, on September 25, 1939, the First World War ended .
Sources:The New York Times (1950 peace date here), Military History, h2g2, German Culture