The Easter Rising, the 1916 armed insurrection that hindsight tells us was the opening act in the successful Irish fight for independence from Great Britain, was by almost any measure a catastrophe.

It did not, at the time, look like the beginning of anything. The conspirators who planned it did not plan well, nor did what plans they laid turn out the way they hoped. Hundreds of people died needlessly.

It would have been almost impossible at the time to predict that the Easter Rising was a turning point in Irish history, that the events of that bloody week would set in motion a chain of events that would ultimately result in Ireland’s independence. Historians and partisans still argue over the efficacy of the revolt and its execution. But Ireland being Ireland, a land that bred some of the finest writing of the last century, it is not surprising that the finest summation of that event comes from a poet, William Butler Yeats, whose ambivalent and mysterious “Easter, 1916” is not only one of the most powerful poems ever written but a splendid snapshot of his nation’s confusion over what had transpired in the revolt and the concurrent understanding that something momentous, a profound game change, had just happened.

When the six-day revolt was over, smothered by fierce British retaliation that left more than 400 people dead—most of them civilians—as well as thousands wounded and the city of Dublin shelled and burned, every aspect of the revolt bore the stench of failure.

A lot of that failure was the fault of the conspirators. They failed to capture key positions in the city of Dublin, including city hall and the docks and railway stations. So when the British sent troops to quell the revolt, they had little trouble entering Dublin, where most of the fighting took place. For that matter, confusion was general all over Ireland.

Worse, the conspirators failed to warn their countrymen about what was happening, so that once the fighting started, some of the fiercest opposition came from the Irish themselves, and not only from the six, largely Protestant counties in the North that would eventually make up what is now Northern Ireland. Many Dubliners, for instance, were confused and baffled by the revolt in their streets, and either actively opposed the insurrectionists or simply refused to help them.

Things might have turned out very differently in the long run had the British settled for merely restoring peace and exploiting that lack of consensus on the part of the Irish. Instead, they savagely put down the revolt and then sent some 90 conspirators to face the firing squad in a matter of days. The reprisals, coupled with the hard line the British took going forward, fueled the opposition and, more important, solidified it. Factions coalesced behind Sinn Fein, the militant group that would spearhead the fight for independence, and the table was set for the civil war that eight years later resulted in the Irish Free State and ultimately in the republic of Ireland in 1937. The Irish lost in the Easter rebellion, but the English lost Ireland.

Yeats was 50 years old at the time, a prominent poet still known mostly as one of the leaders of the Irish renaissance, a movement that extolled the native traditions and folklore of the country. Like his collaborators, the playwright John Millington Synge and Lady Augusta Gregory, Yeats was a cultural revolutionary, but he was not particularly political and disparaged violence as a means of creating an Irish republic. But at the time of the Easter rebellion, he was in the process of changing as a poet, influenced both by literary modernism and the events in his own country. Going forward, he was guided as much by what he saw in the street outside his door as he was by the past, and what he wrote from then on would secure his reputation as arguably the finest poet of the 20th century.

The amazing thing about this transformation is that it did not make the poet more didactic. Yeats was never a preacher. Rather, it made him more subtle, more open to ambiguity. But ambiguity in Yeats’s hands was neither wishy washy nor vague. He might be oblique, but he was never opaque.

In “Easter, 1916,” written in the months that followed the failed uprising, he would express perfectly the confusion and awe with which he and the citizens of his country were consumed.

“We make out of the quarrel with others, rhetoric, but of the quarrel with ourselves, poetry,” he once said, and no poem of his illustrates that sentiment better than “Easter, 1916.” It begins in everydayness: “I have met them at close of day / Coming with vivid faces / From counter or desk among / Eighteenth-century houses. / I have passed with a nod of the head / Or polite meaningless words, / Or have lingered awhile and said / Polite meaningless words, / And thought before I had done / Of a mocking tale or a gibe / To please a companion / Around the fire at the club, / Being certain that they and I / But lived where motley is worn: / All changed, changed utterly: / A terrible beauty is born.”

This is the plainest of the poem’s four verses, but even here, the quotidian is upended and placed in the past tense. That foolish, almost clownish reality (“where motley is worn”) is seen, as it were, in the rear-view mirror. Something has happened, something both terrible and beautiful, and there is no going back.

The rest of the poem proceeds in similar fashion, with people and realities changing like clouds (“Minute by minute they change”), and each part, fractal fashion, reflects the whole of the poem. Even a man he despised he now sees in a different light, less than a hero perhaps but more than a cad.

In the end Yeats is still not sure whether the price paid was worth it (“Was it needless death after all?”). But on one point he does not dither: The men and women he writes about changed history, and in turn they too were changed, as Yeats was, by what happened in that bloody week a century ago.

The easy explanation for all this is to say that the Easter Rising politicized Yeats, and to the extent that it drew him into more complete engagement with his time and his country, that is true. But to stop there does a disservice to the confusion and mystery he has witnessed and set down with such clarity in his poem. For “Easter, 1916” is not only complex and mysterious, it is about complexity and mystery, about beauties that are terrible. Events, especially cataclysmic events, he tells us, are not easily parsed, and we do them and ourselves an injustice to pretend otherwise. All we can do, the poem reminds us, is to confront conflicting realities and reconcile them as best we can. No poet, not even Yeats himself, ever said it better than in “Easter, 1916.”