Portugal did not initially form part of the system of alliances involved in World War I and thus remained neutral at the start of the conflict in 1914. But even though Portugal and Germany remained officially at peace for over a year and a half after the outbreak of World War I, there were many hostile engagements between the two countries. Portugal wanted to comply with British requests for aid and protect its colonies in Africa, causing clashes with German troops in the south of Portuguese Angola, which bordered German South-West Africa, in 1914 and 1915 (see German campaign in Angola). Tensions between Germany and Portugal also arose as a result of German U-boat warfare, which sought to blockade the United Kingdom, at the time the most important market for Portuguese products. Ultimately, tensions resulted in the confiscation of German ships interned in Portuguese ports, to which Germany reacted by declaring war on 9 March 1916, quickly followed by Portugal's reciprocal declaration.

Approximately 12,000 Portuguese troops died during the course of World War I, including Africans who served in its armed forces on the colonial front. Civilian deaths in Portugal exceeded 220,000: 82,000 caused by food shortages and 138,000 by the Spanish flu.