What features make this poem a modernist poem?

Modernism is a term used to classify new and unique characteristics in forms, concepts, and styles of literature, particularly after World War I. "The Second Coming" was written just after World War I, and it was also a reaction to the Irish War of Independence and the Bolshevik Revolution. These were ideological conflicts that questioned the very fabric of civilization. The Bolshevik Revolution was a rebellion, led by Lenin, which dismantled the Tsarist autocracy and led to the rise of the Soviet Union. This revolution followed World War I, another conflict that disrupted established national boundaries and international order.

The poem was also written when Ireland was being torn apart by civil war. Yeats wrote passionately and frequently about conflict, especially conflict in Ireland, and was particularly fearful of mob rule, of the sort that elected totalitarian governments in Russia and Italy.

Throughout his career, Yeats continued to write about the rise and fall of civilizations, lamenting both the inconsistency and disorder of the time period in which he lived and also the bitterness and disillusionment that haunted so many people during this time. This war of ideals—jadedness and love, fear and hope—rose out of a tumultuous time in history, and inspired the tension, confusion, and emotional intensity that characterizes "The Second Coming".