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 in 1916, to which Germany reacted by declaring war on 9 March 1916, quickly followed by Portugal's reciprocal declaration. In total, from 1916 to 1918, around 91 Portuguese ships were sunk and 5 damaged by German U-boats causing at least 170 casualties.
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 by some estimates exceeded 220,000: 82,000 caused by food shortages and 138,000 by the Spanish flu.
1914
July: The German and British Empires enter into secret negotiations over a possible dismemberment of Portuguese Angola; in such a case, most of the land would fall into the hands of the Germans. An Angola-Bund ("Angola League" to promote a German takeover) had been founded in 1912. August to September: Skirmishes occur between German and Portuguese colonial troops in Africa and the Germans instigate tribal revolts. Maziua raid (24 August): A small German force attacked a Portuguese outpost at Maziua in Mozambique, resulting in the first Portuguese casualties of World War I. The German central government accepted full responsibility, and offered an official apology. September: The Portuguese government sends reinforcements to the southern border of Angola. After the war breaks out, the border between German South-West Africa and Angola remains open. The Germans hope to supply food and possibly even arms through it. However, the Portuguese colonial government is hostile and tries to stop all of the trade. A few German nationals in Angola are interned. October: 1600 troops arrive in Portuguese Angola and 1527 troops arrive in Portuguese Mozambique from Portugal, transported by British ships.
1915
November: 1527 troops arrive in Mozambique, commanded by Moura Mendes. The second force is tasked to recapture the Kionga Triangle from the Germans.
1916
When Portugal complies with the British request to confiscate the German ships interned in Portuguese ports, Germany reacts by declaring war on Portugal, thus forcing the Portuguese officially into the war.