The First Iberian War was a protracted conflict fought between Spain and Portugal between 1911 and 1917, which resulted in a million killed on either side and the first true use of aerial and chemical warfare between two rivals. The war is slightly misnamed in that significant portions of its campaigns were conducted in Africa.
The war resulted in a loss for the Portuguese upon the entrance of the French into the conflict, and resulted in the loss of Portugal's West Africa colony, which was promptly divided by France and Spain. However, the war also caused the erosion of the Spanish kingdom's power, leading to the eventual Socialist Revolution of 1927 in Spain. The war is also a significant moment in the gradual movement of Portugal towards an allegiance with the United States, a shift which would become critical during the Cold War.