Obviously there's the recent events in DC shining a light on the intensity of conspiracy nuts in the US, but conspiracy communities in the US have most famously formed around events like the moon landing and JFK assassination. Then while reading through the first several volumes of the Oxford History of the United States, I was struck by the prevalence of conspiracy theories in the politics of the early US as well. Daniel Walker Howe points out that by the 1830s, Americans' "long-standing suspicion of conspiracies against them" had included such targets as the British, the slaves, the Illuminati, the Freemasons, and the banks.
Is this long history of political conspiracy theories unique to the US, or is it a more general phenomenon?
First, I'm going to fuss about the phrase "political conspiracy theories". It is very hard to detangle conspiracy theories from the political; even one that seems relatively politics-free, like the Hollow Earth theory, inevitably ends up intercrossing. (The Hollow Earth theory posits civilizations inside the earth; John Symmes in the early 19th century tried to drum up a visit to the North Pole to go inside and visit. According to the Hollow Earth Research Society in Ontario, the Germans knew about an entrance in Antarctica and that's where Hitler escaped to, and even now, there are Nazis collaborating with the beings who live there.)
A lot of early conspiracy theories are religion-related, but again: how does that get separated from politics, even in the modern age?
So let me rephrase the question to...
Is this long history of political conspiracy theories unique to the US, or is it a more general phenomenon?
To which I can say: no, conspiracy theories are not unique to the US, nor are they even unique to the Western world.
I can say the US has outsized influence in spreading their own conspiracy theories, but that's simply because US has an outsized influence in exporting culture. (This is similar to the "Florida Man" effect -- there seems to be an excess of bizarre criminal behavior from Florida, but transparency laws are what let us learn about them in the first place.) There are buckets aplenty of "local" conspiracy theories both in recent times and in the past.
In Portugal, there is a pseudohistory theory that the Portuguese made voyages to the Americas pre-Columbus. The idea (first formulated by the historian Jaime Cortesão in 1924) is that the Portuguese government kept up a policy of secrecy in order to hide the existence of the Americas and gain advantage.
I wager a fair chance (assuming you are not from Portugal) you haven't heard of this conspiracy until now; it's pretty much only known locally. But it is very well known locally! It is big enough that it has shown up in school textbooks as fact. There is an inscription in Lisbon:
João Vaz Corte-Real – Descobridor da America
that is: João Vaz Corte-Real, Discoverer of America. There are throrough debunkings by Portuguese historians, but the story still persists.
(Again, this is only a 20th century theory. Incidentally, when the actual era in question was happening, there were plenty of conspiracy theories via Portugal, but antisemitic ones.)
It's possible you've heard the "Mandela Effect" conspiracy theory where people remember Nelson Mandela dying in jail; this is supposedly evidence some of us are visitors from a parallel universe. ("Berenstain Bears" vs. "Berenstein Bears" is part of the same thing.)
In South Africa they have their own "Nelson Mandela died" theory, but since he was President of South Africa and they do remember that, it's a bit different. Supposedly in 1985 Mandela died and was replaced by an imposter: Gibson Makanda. This may or may not have been the work of the Illuminati, and it means, multi-dimensional-chess style, that the one who led the African National Congress against the National Party was actually a puppet of the National Party.
Some of the conspiracy theories are hard to "translate" for other cultures. In Java (one of the islands of Indonesia) in 1998, there was a bizarre set of hundreds of incidents where people dressed as ninjas were knocking on doors, leading to a mass hysteria that they were sorcerers. (Or hunting sorcerers; conspiracy theories can be contradictory.) There was a series of accusations of witchcraft, culminating in 120 people being massacred.
Conspiracy theories are not unique to any place, they just take on their local character. Of course, circumstances across time and space can have uncanny resemblances, so let's do one more--
There was an outbreak of plague in Marseilles, France. England went under quarantine, using the advice of the doctor Richard Mead. Affected areas were to be sealed off for 20 days. Paranoid conspiracy theories about the over-reach of government becoming a "dictatorship" started to spread; some thought the British were in fact immune. People started to doubt the medical experts and turn to the words of religious leaders.
This was 300 years ago, in 1720.
...
Selected Reading:
Hollow Earth: Standish, D., Standish, M. (2006). Hollow Earth: The Long and Curious History of Imagining Strange Lands, Fantastical Creatures, Advanced Civilizations, and Marvelous Machines Below the Earth's Surface. Da Capo Press.
Portugal discovering the Americas early: Fritze, R. (1994). The Pseudohistory of Who Discovered America. Skeptic, Vol. 2, No. 4.
Portgual and antisemitism: Soyer, F. (2019). Antisemitic Conspiracy Theories in the Early Modern Iberian World. Journal of Jesuit Studies, 6(3), 512-515.
Nelson Mandela imposter: Shoki, W. (2020). On conspiracy theories. Africa Is a Country.
Java and the sorcerer ninjas: Siegel, J. T. (2006). Naming the Witch. United States: Stanford University Press.
England under quarantine: Krischer, A (2020). History does not repeat itself? How an early modern epidemic led to conspiracy theories and religious punishment fantasies. University of Münster (working paper).