Portugal entered World War One on the Allied side on this day in 1916.
We don't think much of Portugal's role in the war, but one of them would be take part in the combat in Africa, which we also tend not. A good thread (I think there are actually several) on this topic is found on the excellent Society of the Military Horse website.
Portuguese soldiers disembark in France,December 1916.
Twelve Thousand Portuguese soldiers died in World War One, including African soldiers serving Portugal in Africa. That's a small number compared to most other combatants, but it was a significant number in the context of their role in the war. The war, additionally, caused food shortages at home resulting in 82,000 deaths. The Spanish flu killed an additional 138,000, but that likely would have occurred anyhow.
Portuguese solders leaving for Angola.
This date is a bit confusing, as technically Portugal had been at war since March 9, when Germany declared war on it due to it having confiscated German ships in Portuguese ports upon request of the British. Portugal had a long running friendly connection with the United Kingdom dating back to the Napoleonic Wars. This understandingly provoked a German reaction. Portugal replied with its own declaration of war and began to organize its forces for Western Front action. It did not immediately send troops however. Still, a war aim of its own, the return of Kionga Mozambique from German occupation, occupied since 1894, was incorporated in the Allied war aims by June 9, 1916.
Portuguese soldiers loading a mortar. In combat the Portuguese in Europe resembled the British in appearance from whom they had secured some equipment.
On this date the Portuguese accepted a British invitation to actually participate in the war, which may seem odd, but it required the act of the Portuguese parliament. It's participation in the war in Europe was always limited and in some ways its participation in Africa was more significant. Perhaps the most famous event associated with Portugal and World War One did not involve combat at all, however, and was the apparitions at Fatima, which commenced with the visitation of an angel in the Spring of 1916, and which continued in May 1917 with the visitation of Mary. The final apparitional phenomenon was the October 1917 Miracle of the Sun which was witnessed by a large number of people.