"A Terrible Beauty Is Born"

Context: On Easter Sunday of 1916, while England was at war with Germany, the Irish Nationalists attempted an insurrection against British rule.

The insurrection was put down after bitter street fighting, and many of the Irish were later executed because the movement for their independence was secretly supported by Germany. Yeats knew many of these men, and this poem is a description of and a kind of requiem for them. The quotation above–which appears in slightly varying form throughout the poem–refers to the change that came over these patriots when they gave themselves to their cause. The first stanza reads:
I have met them at close of day
Coming with vivid faces
From counter or desk among grey
Eighteenth-century houses.
I have passed with a nod of the head
Or polite meaningless words
. . .
Being certain that they and I
But lived where motley is worn:
All changed, changed utterly:
A terrible beauty is born.