“Twenty centuries of stony sleep
Were vexed to nightmare by a rocking cradle
And what rough beast, its hour come round at last,
Slouches towards Bethlehem to be born?”
In the poem, the twenty centuries have already elapsed,
and from the perspective of 1919, the spectre of looming
cataclysm would have to be an adumbration of WWII,
the way I see it.
As I understood it, Yeats felt that history moved in two thousand year cycles. He dated the start of ancient history to the time when Leda was raped by Zeus, who descended as a bird (a swan). This ultimately led to Helen, and the Trojan war, and a whole religion of the gods.
And he felt our history started when Mary was impregnated by the Holy Spirit, who descended as a bird (a dove). This lead to Christianity. Now, Yeats was thinking about what cycle of history would be coming next.
I could be wrong, but that's what I learned in my studies of Yeats and it makes sense to me.
But like I say, it's great to hear other views as well, and maybe your views are correct! Since the poem was written in 1921, he'd have to be quite prophetic to foresee the rise of fascism and the storm clouds of WWII. If he had contemporary events in mind, I suspect he was thinking about Ireland and Irish independence, which was a burning hot issue at that time.
If you don’t mind one other comment, reading the poem today one can sense that mysterious, malevolent beast in the poem might represent militant Islam. I doubt Yeats had Islam in mind specifically, but he might have sensed how the aftermath of WWI was giving rise to all kinds of dangerous new ideologies — militant Islam was one of those (the Muslim Brotherhood was founded in the 1920's), and it's been one of the most enduring.