Nigeria, Dec. 19 -- A grim anniversary passed this year almost unnoticed.

Ninety years ago, in October 1935, Fascist Italy invaded Abyssinia - one of only three African states, alongside Liberia and South Africa, to retain formal sovereignty after the imperial partition of the continent. What followed was not merely a colonial war. It was a decisive test of whether the international system created after the First World War would defend its own rules when a powerful state violated them.

Abyssinia fought back with courage and endurance. Poorly armed, with almost no air force, its forces resisted for months against a modern European army. Italian aircraft bombed villages and troops at will. Mustard gas was sprayed from the air in flagrant ...