I’ve found an open access PhD thesis titled Prophets of Decline (2003), which has two chapters relevant to understanding the historical context for Lovecraft’s reception of Spengler in the America of the later 1920s…

Once returned to Providence…

Lovecraft began in the late 1920s to develop his notions of the decline of the West — notions that his reading of Oswald Spengler’s great work on the subject only helped to clarify and develop. (S.T. Joshi, I Am Providence).

From a Lovecraft letter of 1926…

Recently I saw a review of Spengler’s ‘Decline of the West’ — which will make splendid discussion-matter with Mortonius [James Morton]. Did you see it — in the New York Post?

This must be the front-page review by the anti-communist John Cournos, “Is Our Civilization Doomed? No Chance Of Survival — Says Spengler” The New York Post Literary Review supplement, 29th May 1926. (Now un-findable online, and it seems there’s no microfilm of this title at libraries before 1934?).

The Prophets of Decline thesis thus offers what are effectively two ‘free bonus’ appendices, in a digestible thirty pages, for readers of S.T. Joshi’s book on Lovecraft’s intellectual life The Decline of the West (now a very affordable and cleanly formatted ebook on Amazon). The chapters are quite dense and have some typos, but are admirably concise and focused. They outline Spengler’s initial reception in America, and then the changed perceptions there of Spengler in the 1930s — as the civil war within socialism raged and both communism and fascism twisted the ways he was portrayed and understood. Part of the problem on the right was that Spengler did not endorse Hitler. He had also supported those purged in 1934, and because of this was subject to a campaign of vilification by the Nazi Party.

As for the rest of the thesis it tells the larger story of the reception by journalists and intellectuals of the alarmist doom-mongers of 1896-1961, and as such provides useful background for better understanding the doom-mongers of the 1970s and 80s.