On 7th August, 1914, President de Arriaga of Portugal declared his support for the Allies. At this time, Portugal had over 33,000 men in its army. By November, 1914, Portuguese troops were involved in skirmishes with German troops on the frontier between Mozambique and German East Africa. However, this did not provoke a full-scale declaration of war.

In February, 1916, the Portuguese government ordered its navy to seize German ships in its harbours. Germany responded by declaring war on Portugal. About 100,000 Portuguese eventually fought with the Allies on the Western Front and in Mozambique. The army suffered 21,000 casualties, including over 7,000 dead.