With Portugal's entry into the Great War, the Navy was given a difficult role as it had to defend the colonies, coasts, and Atlantic ports, protecting the shipping lines between national ports and the Portuguese merchant navy. To fulfill its obligations, It deployed between actions at sea and land, counting on an effective, a force of about 4000 men (about 300 officers and 3700 sergeants and enlisted men).
At the coastal level, priority was given to the defense of the ports of Lisbon, Setúbal and Leixões, through the installation of dams for submarine defense, the creation of patrol squadrons made up of Navy ships and submarines, complemented by a large number of vessels.
Mine dredging was carried out in the ports and access bars to these, defensive mining was carried out in the port of Lisbon and piloting for the entrance to the bar. Surveillance posts and incipient shore batteries were also installed on land to defend the bars.
On the Algarve coast, the Esquadrilha Fiscal da Costa, consisting of a gunboat, two tugs and three steamers, patrolled the Algarve coast with difficulty, requiring the help of French and English naval forces for this purpose.
In addition to this foreign aid in the Algarve, the meager Portuguese resources led to the creation of a French naval base in Leixões, which patrolled the northern coast of the country and the creation of a French naval aviation center in Aveiro, which operated under the same.
The creation in 1917 of the Armada Aviation Service in Lisbon made it possible to carry out aerial patrols with seaplanes to locate mines and submarines on the surface.
In the Azores, the defense of the port of Ponta Delgada was up to a warning, a destroyer and a naval gunboat, together with two shore batteries and assisted with the installation of a US base in Ponta Delgada.
In Africa, two gunboats were stationed in Cabo Verde (Cape Verde) and two batteries were installed on land. The defense of Mozambique was carried out by a cruiser, a gunboat and two gunboats, which were involved in the conflict in the north of the territory along the the Rovuma River.
On land, the Navy contributed to the constitution of two expeditionary battalions destined for Africa, one for Mozambique, which fought the Germans on the northern border, and the other for Angola, where it faced native populations armed by the Germans.
The Navy also contributed to the escort of national merchant ships and transport of troops to the colonial territories and France and, despite all the difficulties, managed to organize during the conflict a total of 148 convoys destined for Africa.