The poetical supplement of Wittgenstein's Tractatus would begin "History is everything that happens." (Sometimes I think it should begin with something funnier, like "History is everyone who's on my case." This has the virtue of correctly transposing "everything" into "everyone", i.e., things into persons.) Meanwhile, the "secret project" of Chapter 19 (pp. 4-6) in Spring in All, a "final and self inflicted holocaust" (remember this is published in 1923), "the annihilation of every human creature on the face of the earth", marks the historical moment of Williams' poetical imagination. Now, compare:
Houses crumble to ruin, cities disappear giving place to mounds of soil blown thither by the winds, small bushes and grass give way to trees which grow old and are succeeded by other trees for countless generations. A marvelous serenity broken only by bird and wild beast calls reigns over the entire sphere. Order and peace abound.
...There, soul of souls, watching its own horrid unity, it boils and digests itself within the tissues of the great Being of Eternity that we shall then have become. (S&A, p. 6)
Here it can be seen that solipsism, when its implications are followed out strictly, coincides with pure realism. The self of solipsism shrinks to a point without extension, and there remains the reality coordinated with it. (T5.64)
Wittgenstein is not defending solipsism. In the Investigations he explicitly pushes back against the view presented here. And Williams is not, of course, proposing a glorious holocaust of humanity! He is rejecting History (with a capital H) as the situation of poetry and proposing imagination instead. When Wittgenstein later rejects his point of departure, "the world", in favor of a much more local situation ("this lamp", "this tree") he is learning Williams' lesson: "no ideas but in things". (Though as I've argued before this, too, can be made more pangrammatically precise.) We must begin with actual pictures of actual facts.
Philosophy must reject a radical solipsism in order to avoid its implication: an inhumanly "real" world comprising "everything that is the case". Poetry, meanwhile, must reject the idealism of each person's solidarity with all human beings. (History is everyone that's on my case, indeed!) Such idealism is at once revolutionary and suicidal. Williams' nightmare vision of a "final solution" is apt.