I'm sure everyone has stopped reading this thread, but just to respond to those who took the time to reply to my letter:

@Damianus. I don't disagree that, as you say, "there was nonetheless a large and thriving native population throughout the Mississippi valley when Europeans arrived." The question is: large in comparison with what? Were there large cities, as in Mexico; towns, as in the Yucatan and Caribbean; villages, as in southern Central America; or isolated hamlets and scattered farms -- as, apparently, in the current US?

It is not in the least surprising that Soto and/or his companions would have described Native American populations in the Mississippi valley as "very well peopled with large towns... two or three of which were to be seen from one town." Soto's expedition, like virtually all Spanish "conquistador" expeditions of the 15th and 16th centuries, was not a government-financed exploit manned by soldiers in the Spanish army. No, these were privately financed expeditions (the fact that Spanish kings or queens sometimes invested their private assets does not change this fact) and were put together and manned by non-professionals who often as not picked up soldiering along the way.

After the first wave of "conquistadors" had conquered the densely settled regions now called Mexico and incorporated them into the Spanish Empire, the main objective of late-comers was to find a "new Mexico" (in Spanish: nueva, not nuevo, Mexico, meaning "a new City of Mexico"). The latter-day, would-be “conquistadors” searched high and low through North America for such a city; the best they came up with was the area we still call New Mexico (originally: Nueva Mexico).

The two ways that the “conquistadors” (better: entrepreneurs and venture-capitalists) had of recouping their investment in their expeditions were: 1) to luck out and find (i.e. steal) a treasure such as Motecuhzoma's or Atawallpa's; or, much more often, 2) to convince others that they really had discovered a "rich province" that, Ponzi-scheme style, was worth further investments. Since, as I am arguing, the only source of wealth other than precious metals themselves was population (labor), they often -- no, ALWAYS -- exaggerated the claims of populous towns, cities, etc. If there really were large 16th-century cities in the Mississippi valley, there would be archaeological remains; instead, we have descriptions from one of Soto's men.

Finally, the fact that, as you write, “William Bradford, first governor of the Massachusetts Bay Colony, candidly noted in his journal ‘The good hand of God favored our beginnings by sweeping away great multitudes of the natives... that he might make room for us...’” hardly contradicts my basic point. If there had not been, in Bradford’s own words, “great multitudes of the natives” (a slight exaggeration, but let it pass) in Massachusetts to begin with, the Puritans would have starved to death their first winter in 1608, as is well known. (It is also noteworthy that Bradford, the good Puritan, also thanks God for slaughtering the Puritans’ rival English settlers, thus showing who He favors.) The English in Massachusetts employed Native Americans as labor for much longer than is generally recognized (well into the 19th century), as Bradford himself hints at at several points in the same text. But beyond that, the Puritans built their farms on the abandoned farms of Native Americans, and used Native American crops, etc. Without the prior indigenous settlement of “New England,” the region’s later resettlement by English and other European settlers would have been much more difficult and protracted. Later in the same text, Bradford also gives thanks to God for “giving such plenty of all manner of food in a wilderness” -- note, this “wilderness” had long been domesticated by Native American ingenuity, but in such a form that Bradford did not recognize it as cultivated -- "insomuch that... all sorts of English grain, as well as Indian, are plentiful amongst us.”

As for what happened after 1763: by then, 271 years after the first European colonization of the Americas (n.b., only 246 years have passed SINCE 1763), new technologies were in play. Native American settlements were still very important (much more than you would guess by reading only the standard US histories), but no longer strictly determinative.