The Second Italo-Ethiopian War, also referred to as the Second Italo-Abyssinian War, was a war of aggression waged by Italy against Ethiopia, which lasted from October 1935 to May 1936. In Ethiopia it is often referred to simply as the Italian Invasion (Amharic: ጣልያን ወረራ, romanized: Ṭalyan warära), and in Italy as the Ethiopian War (Italian: Guerra d'Etiopia). The war is regarded as the largest colonial campaign in history.

On 3 October 1935, two hundred thousand soldiers of the Regio Esercito (Royal Army) commanded by Marshal Emilio De Bono attacked from Italian Eritrea without prior declaration of war. At the same time a smaller force under General Rodolfo Graziani attacked from Italian Somalia. On 6 October, Adwa was conquered, a symbolic place for the Italian army as it was the site of their defeat at the Battle of Adwa during the First Italo-Ethiopian War. On 15 October, Italian troops seized the town of Aksum, and its famous obelisk was torn from its site and sent to Rome to be placed symbolically in front of the building of the Ministry of Colonies.

Exasperated by De Bono's slow and cautious progress, Benito Mussolini replaced him with General Pietro Badoglio. Ethiopian forces launched a counterattack in December 1935 that achieved some gains and temporarily halted the Italian advance. Frustrated, Mussolini authorized the use of mustard gas, with the Regia Aeronautica (Royal Air Force) dropping 330 tons over the next four months, violating the 1925 Geneva Protocol. The Italians resumed the offensive in early February, decisively defeating the Ethiopian army at Amba Aradam, Tembien, and Shire. On 31 March 1936, the Italians won a decisive victory at the Battle of Maychew, which nullified any possible organized Ethiopian resistance. Emperor Haile Selassie was forced to flee into exile on 2 May, and Badoglio's forces arrived in the capital Addis Ababa on 5 May. Mussolini then announced the annexation of Ethiopia on 9 May and the colony of Italian East Africa was formed. Hostilities did not cease with the end of conventional war operations and Ethiopian resistance to Italian rule would continue until the country was liberated in 1941.

Overall, the war in Ethiopia was a major military success for the fascist regime in Italy, achieved relatively quickly and with considerable propaganda value. However, it also placed a significant strain on an already seriously unbalanced budget, weakening Italy's long-term economic position. The Italian aggression against Ethiopia had significant diplomatic consequences, leading to the end of the Stresa Front and Italy's rapprochement with Nazi Germany. Simultaneously, the inability of the League of Nations to prevent the invasion discredited it internationally on the eve of the Second World War.

Italian troops used mustard gas in aerial bombardments (in violation of the Geneva Protocol and Geneva Conventions) against combatants and civilians in an attempt to suppress Ethiopian resistance. Deliberate Italian attacks against ambulances and hospitals of the Red Cross were reported. By all estimates, hundreds of thousands of Ethiopian civilians died as a result of the Italian invasion, which have been described by some historians as constituting genocide. Crimes by Ethiopian troops included the use of dumdum bullets (in violation of the Hague Conventions), the killing of civilian workmen (including during the Gondrand massacre) and the mutilation of captured Eritrean Ascari and Italians (often with castration).

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