At the start of WWI Portugal was a neutral party, but they were in a fairly precarious position to maintain that neutrality for long.

One point of conflict was Angola and Mozambique, which were Portuguese colonial possessions in Africa. Germany had a fairly paltry empire at the start of WWI, which was a point of contention between them and the other major European powers (e.g. Britain, France, Spain, The Netherlands, etc. all of whom had substantial valuable empires). German forces fought against Portuguese colonial troops in Africa during 1914 and 1915, so already Portuguese neutrality is in question.

Add to that the extensive trade between Portugal and Britain coupled with unrestricted U-Boat warfare and the attempted German blockade of Britain at sea and you have a very fraught situation. In the end in 1916 Portugal would prevent many German and Austria-Hungarian ships from leaving the port of Lisbon due to a request from Britain to do so, this resulted in an immediate declaration of war from Germany.

At the end of the war Portugal receives minor territorial concessions in Africa (the port of Kionga (Quionga) in present day Mozambique).