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World War I

In 1914, the German Empire formulated plans to usurp Angola from Portuguese control. Skirmishes between Portuguese and German soldiers ensued, resulting in reinforcements being sent from the mainland. The main objective of these soldiers was to recapture the Kionga Triangle, in northern Mozambique, the territory having been subjugated by Germany.In 1916, after Portugal interned German ships in Lisbon, Germany declared war on Portugal. Portugal followed suit, thus entering World War I. Early in the war, Portugal was involved mainly in supplying the Allies positioned in France. In 1916, there was only one attack on the Portuguese territory, in Madeira. In 1917, one of the actions taken by Portugal was to assist Britain in its timber industry, imperative to the war effort. Along with the Canadian Forestry Corps, Portuguese personnel established logging infrastructure in an area now referred to as the " Portuguese Fireplace ".

Throughout 1917 Portugal dispatched contingents of troops to the Allied front in France. Midway in the year, Portugal suffered its first World War I casualty. In Portuguese Africa, Portugal and the British fought numerous battles against the Germans in both Mozambique and Angola. Later in the year, U-boats entered Portuguese waters again and once more attacked Madeira, sinking multiple Portuguese ships. Through the beginning of 1918, Portugal continued to fight along the Allied front against Germany, including participation in the infamous Battle of La Lys. As autumn approached, Germany found success in both Portuguese Africa and against Portuguese vessels, sinking multiple ships. After nearly three years of fighting (from a Portuguese perspective), World War I ended, with an armistice being signed by Germany. At the Versailles Conference, Portugal regained control of all its lost territory, but did not retain possession (by the principle of uti possidetis) of territories gained during the war, except for Kionga, a port city in modern-day Tanzania.

Portuguese territories in Africa eventually included the modern nations of Cape Verde, São Tomé and Príncipe, Guinea-Bissau, Angola, and Mozambique.