On October 3, 1935, Italian troops crossed the Mareb into Ethiopia from Eritrea and moved into the Ogaden from Somalia, with no declaration of war. Marshal Emilio De Bono had about 100,000 men in the north. The Italians had aircraft, armour and mustard gas. The Ethiopian army, perhaps 200,000 strong on paper, had rifles, some of them from the 1880s, and almost no artillery and no radios.
The casus belli was Walwal, a clash in December 1934 at a desert waterhole between an Italian garrison and an Ethiopian escort that left about 150 Ethiopians and 30 to 50 Italians dead. Haile Selassie asked the League of Nations to arbitrate. Mussolini invaded anyway. He spent the intervening months moving about 400,000 troops into Eritrea and Somalia and building the roads that would carry them.
The League did what it was built to do and then did not. It declared Italy the aggressor on October 7 and voted sanctions in November, and the sanctions excluded oil, coal and steel, because the British and French governments calculated that including them would push Mussolini into an alliance with Hitler. The Hoare-Laval plan of December 1935, which proposed handing Italy most of Tigray and Ogaden, was leaked. Hoare resigned.
The war was decided by gas and air power. The Italians dropped mustard gas from February 1936, on a scale estimated at 300 to 500 tons, and Red Cross units in the field were reporting it at the time. The northern front collapsed after Amba Aradam in February, and the last Ethiopian army was destroyed at Maychew in March. Haile Selassie left for exile on May 2. The city fell in May.
Italy declared the country part of Italian East Africa and held it for five years, until British and Ethiopian forces took Addis Ababa in May 1941. Haile Selassie had spoken to the League in Geneva in June 1936 and been ignored, and his warning that it would happen to others too is why the episode is remembered. The failure over Ethiopia is usually taken as the point at which collective security stopped being credible.