I tried to do this in my
Tripartite Alliance Earth timeline of two decades ago (!), beginning with a United States that took a decidedly xenophobic swing in the early 19th century (earlier, and longer, participation in the Napoleonic Wars) eventually to go down an Argentine trajectory of underperformance leading to seriously negative political sequelae. This included, by the 1970s, a descent into a messy and chaotic police state that was starting to get involved in all kinds of conflicts with neighbours near and far.
Leaving the general plausibility of my timeline aside, I'm not sure that this outcome is stable. How do you get a United States that is not only sufficiently isolationist and militaristic, but a) keep it there and b) manage to get it not overwhelm its neighbours? In the specific case of TpA, that United States had underperformed significantly relative to OTL, with particularly South America doing well, such that the United States was only one relatively large and wealthy country of many, something that could be counterbalanced by its neighbours. Even then, I didn't think it plausible to assume that this United States would be solidly on side with this aggressive militarism, if only because it was a large federation with presumably plenty of people disinterested in autarkic militarism. How do you get Americans not to care about their own freedom?
In the case of TpA, the United States ended up falling apart. In its setting--developed in a free-form collaborative storytelling forum, FWIW--if it hadn't fallen via external war it would have fallen via internal dissension.