The poem was written in 1919 in the aftermath of the First World War and the beginning of the Irish War of Independence that followed the Easter Rising, at a time before the British Government decided to send in the Black and Tans to Ireland. Yeats used the phrase "the second birth" instead of "the Second Coming" in his first drafts.The poem is also connected to the 1918–1919 flu pandemic: In the weeks preceding Yeats's writing of the poem, his pregnant wife Georgie Hyde-Lees caught the virus and was very close to death. The highest death rates of the pandemic were among pregnant women—in some areas, they had up to a 70 percent death rate. While his wife was convalescing, he wrote "The Second Coming."
Those two lines strike me as particularly apropos for our times. Maybe it has always been so... but maybe not.