Actually, I think the trend toward greater conservatism in the United States is a good bit older than WWI, and is somewhat common in colonial societies. I'm using colonial in a very special way, here, to refer to societies settled by emigrants from elsewhere.
It's something of a truism that immigrant groups in the United States have cultures and attitudes that resemble those in the old country from the time they left. A lot of Scandinavians, for example, see their American cousins as a sort of time capsule from the 19th century. And, the case with Irish Americans is even more notable, in that they in many cases preserve religious practices and nationalist attitudes dating from 1916.
Not all colonial societies are so conservative, of course. Australia is probably closer to many modern European attitudes for example. OTOH, you can see the same sort of cultural conservatism among the Afrikaners, who are even more set in 17th century modes of thought than Americans, British South Africans, who are still rooted in the 19th century British Empire, and in Brazil, which preserves an archaic form of Portuguese.
America was initially settled in the 17th century, and that is where the roots of American culture still lie. American English, for example, preserves many 17th and 18th century pronounciations and constructions that have become obsolete in the UK. American Protestantism is essentially 17th century Protestantism, modified by the ideals of the American Revolution. That Revolution is often misunderstood. While many of its roots were in the thought of the 18th century Enlightenment, it had deeper roots in the political theories of the Commonwealthists, who were writing immediately after the Glorious Revolution of 1688.
The American Constitution rooted 17th and 18th century governmental models in the American body politic, and these models have been very stable ever since. American political discourse has largely been rooted in 17th and 18th century concerns and perspectives ever since.
This has meant that American and European cultures started to diverge by the beginning of the Romantic period, and have become more and more alien ever since. While a taste for Romantic style did cross the Atlantic, the writing of American "Romantics" like Hawthorne are visibly shaped by a 17th century concern with issues of salvation, sin, and redemption, albeit often in metaphorical or somewhat secularized form. Such important Romantic ideas as tribal nationalism, sturm und drang, occultism, and so on never influenced the culture of the United States as they did European culture. Nor did Socialism ever have the same kinds of roots in American national experience as it did in most European countries. The view of Socialism as an alien creed is what drives the American phobia of Socialism and the Left generally. Likewise, Neitzsche, whose influence on European culture has been incalculable since 1900, if not before, was usually regarded by American intellectuals with a great deal of suspicion until at least the 1960s.
In many respects, American and European cultures were more divergent in the year 1900 than they are today. While Europe was still Victorian in 1900, it was beginning to move toward a decadence, lushness, and sensuality that has since become the dominant flavor of European culture. That, depending on one's perspective, Europeans were irreligious sensualists, or Americans hectoring prudes, was already a part of Euro-American interactions in 1900. Likewise, while religion was still strong in Europe, the majority of European intellectuals were secular, often atheist, by 1900. This was much less true in Britain, and not true at all in America. European art at the turn of the 20th century was moving toward Expressionism, European music toward the kind of musical Expressionism found in Stravinsky. American art and music, however, were radically different, and much more conservative.
American popular culture is an exception to this, but less an exception than is often supposed. Country music, for example is rooted in early ballad styles. Blues, Jazz, and Rock music, to name but three, have come from a group marginalized and oppressed within American society. Radical or experimental literature in America almost always comes from one kind of minority or another, while the literature of more mainstream American writers shows the perspective and even styles of the age of Defoe.
During the World Wars, these differences were masked by common historical experiences, and common struggles. However, since the end of the Cold War, they have come again into the open. After the World Wars Europe continued to change rapidly, moving along a secularising, socialistic, and humanitarian trajectory that has led Europeans to both very pleasant societies, and a certain international weakness. The United States has also changed, but more slowly, and in accord with its own internal dynamics and deep-rooted attitudes.
In this regard, it might be better to see America not as a Western society, but more as an outlier of Western culture that took a different turn in the 17th and 18th centuries. In this sense, the relationship between Europe and America is an example of alternate history in the real world.
