The Portuguese Expeditionary Corps (Corpo Expedicionário Português or CEP) was the main military force from Portugal

Portugal

Portugal , officially the Portuguese Republic is a country situated in southwestern Europe on the Iberian Peninsula. Portugal is the westernmost country of Europe, and is bordered by the Atlantic Ocean to the West and South and by Spain to the North and East. The Atlantic archipelagos of the...

 that participated in the First World War. Portuguese neutrality

Neutrality (international relations)

A neutral power in a particular war is a sovereign state which declares itself to be neutral towards the belligerents. A non-belligerent state does not need to be neutral. The rights and duties of a neutral power are defined in Sections 5 and 13 of the Hague Convention of 1907...

 ended in 1916 after the seizure of German merchant ships resulted in Germany declaring war. The expeditionary force

Expeditionary warfare

Expeditionary warfare is used to describe the organization of a state's military to fight abroad, especially when deployed to fight away from its established bases at home or abroad. Expeditionary forces were in part the antecedent of the modern concept of Rapid Deployment Forces...

 was raised soon after and included around 55,000 soldiers.
At the outbreak of the First World War, Portugal had declared its neutrality.