Conventional wisdom has long held that the resumption of large-scale American beer production after Prohibition led to the use of 'adjuncts' not typically found in European beer—most notably, corn, rather than a full grain bill of barley or a combination of barley and wheat. These cheaper ingredients led to a bland, watered-down brew, and until the resurgence of craft beer in the US over the past 20 years, it was impossible to find a beer made with ingredients that America's first European settlers would have recognized; at least, that's how the story goes.
The truth is a bit more complicated, and it goes back to the Mayflower, which is famously said to have chosen its landing spot when the beer ran out. While that part of the story may seem far-fetched, it's based in fact.
Let's rewind to 1620: the Pilgrims had been at sea for nearly two months and were off-course by the time they sighted land. While they had set a course for the Virginia colony (already established on a permanent basis some 13 years before), they had ended up in New England, and they had a problem—they were running out of beer.
Laugh all you want, but this was no small matter—water aboard ship was likely to become brackish and potentially deadly, while beer remained drinkable. Captain Christopher Jones recognized the need to preserve the dwindling stocks for his sailors on the return journey (which would be far too dangerous to undertake until the following spring), and so the passengers were encouraged to land near the top of Cape Cod.
These instructions did not go down well with the Pilgrims; William Bradford complained that he and his companions "were hastened ashore and made to drink water, that the seamen might have the more beer."
Once on land, the new settlers remained suspicious of the local water, given that few (if any) were accustomed to drinking fresh water, and unrest reigned. Bradford was called upon to negotiate with the captain, who remained anchored in the harbor with his crew for the winter, but he was rebuffed and told that he would not receive more beer "not even if he were their own father." But with the harsh weather conditions, the Pilgrims were back aboard ship very shortly—and on Christmas day they were even allowed some of the beer, although half the settlers would still die over the course of that first winter.
So far, so good—but beer writer Bob Skilnik notes that beyond Bradford and some other colonists' writings, the story isn't really mentioned again until the 1930s, when Budweiser begins to advertise their post-Prohibition product with a slightly dressed-up version of the tale. The tale became a regular feature of Bud's Thanksgiving advertising through the 1940s, and so perhaps became somewhat blown out of proportion, but there is still more than a kernel of truth to this myth, although we should view the lack of within the context of a lack of supplies in general.
However, the notion of the Pilgrims enjoying a dark, English-style ale with the local Native Americans (who seem often to figure as afterthoughts in the story, despite being the only people who actually understood how to raise food in the New England climate) at the first Thanksgiving contains considerably more artistic license—and that brings us back to the notion that Prohibition's after-effects shoehorned corn into American beer.
In fact, corn was used from the beginning of English settlement for brewing, both at Jamestown and Plymouth; barley crops often failed, and the cost of importation (even after the settlers stopped dying in such alarming numbers) was prohibitive, although it should be noted that early Dutch settlers in what is now New York and Delaware had more success in brewing something that looks more like a European beer, with hops and malted barley.
The Dutch also had a functioning brewery in what is now Lower Manhattan by 1613—beating the Mayflower immigrants, who would not have anything resembling a formal brewhouse until at least 1621, to the punch.
Even before that, the Roanoke colony tried brewing with corn as early as 1584 (obviously before going missing), so there was already some awareness that it would be problematic to rely on barley alone. Captain George Thorpe, around 1620, described the difficulties of growing and malting barley in Virginia, largely brought on by the summer's heat, but he conceded that "Wee have found a waie to make soe good drink of Indian corne I have divers times refused to drinke good stronge English beare and chose to drinke that."
Corn proved useful for both beer and liquor production, and while we've previously mentioned the very wide range of ingredients used in early American beer production, corn remained a staple that helped round out a grain bill, even once barley became more readily available.
Later waves of immigrants, including the German brewers who would become business titans until Prohibition put many of them out of business, continued to use corn in their recipes—obviously, the Reinheitsgebot was not a major concern for many of them.
After Prohibition, many surviving brewers simply revived their earlier recipes, corn and all, and while there were certainly changes in the beer industry (and brewing) in the years that followed, the use of corn was not a new way to make a cheaper product—its roots go back to the earliest American recipes.
Beer has been a part of American life since the first European settlers arrived—and corn for a considerably longer period for the land's first inhabitants—so there's no shame in enjoying a beer that includes corn in its recipe over Thanksgiving; it's an American tradition.