Disunion follows the Civil War as it unfolded.

Of all the bedtime-story versions of American history we teach, the tidy Thanksgiving pageant may be the one stuffed with the heaviest serving of myth. This iconic tale is the main course in our nation’s foundation legend, complete with cardboard cutouts of bow-carrying Native American cherubs and pint-size Pilgrims in black hats with buckles. And legend it largely is.

In fact, what had been a New England seasonal holiday became more of a “national” celebration only during the Civil War, with Lincoln’s proclamation calling for “a day of thanksgiving” in 1863.

That fall, Lincoln had precious little to be thankful for. The Union victory at Gettysburg the previous July had come at a dreadful cost – a combined 51,000 estimated casualties, with nearly 8,000 dead. Enraged by draft laws and emancipation, rioters in Northern cities like New York went on bloody rampages. And the president and his wife, Mary, were still mourning the loss of their 11-year-old son, Willie, who had died the year before.

So it might seem odd that Lincoln chose this moment to announce a national day of thanksgiving, to be marked on the last Thursday in November. His Oct. 3, 1863, proclamation read: “In the midst of a civil war of unequaled magnitude and severity … peace has been preserved with all nations, order has been maintained, the laws have been respected and obeyed, and harmony has prevailed everywhere, except in the theater of military conflict.”

But it took another year for the day to really catch hold. In 1864 Lincoln issued a second proclamation, which read, “I do further recommend to my fellow-citizens aforesaid that on that occasion they do reverently humble themselves in the dust.” Around the same time, the heads of Union League clubs – Theodore Roosevelt’s father among them – led an effort to provide a proper Thanksgiving meal, including turkey and mince pies, for Union troops. As the Civil War raged on, four steamers sailed out of New York laden with 400,000 pounds of ham, canned peaches, apples and cakes – and turkeys with all the trimmings. They arrived at Ulysses S. Grant’s headquarters in City Point, Va., then one of the busiest ports in the world, to deliver dinner to the Union’s “gallant soldiers and sailors.”

This Thanksgiving delivery was an unprecedented effort – a huge fund-raising and food-collection drive. One soldier said, “It isn’t the turkey, but the idea we care for.”

The good people of nearby Petersburg, Va., had no turkey. Surrounded and besieged by Grant’s armies since June, they were lucky to eat at all. The local flocks of pigeons had all mysteriously disappeared and “starvation parties” were a form of mordant entertainment in this once cosmopolitan town.

What prompted Lincoln to issue these proclamations – the first two in an unbroken string of presidential Thanksgiving proclamations – is uncertain. He was not the first president to do so. George Washington and James Madison had earlier issued “thanksgiving” proclamations, calling for somber days of prayer. Perhaps Lincoln saw an opportunity to underscore shared American traditions – a theme found in the “mystic chords of memory” stretching from “every patriot grave” in his first inaugural.

Or he may have been responding to the passionate entreaties of Sara Josepha Hale, editor of Godey’s Lady’s Book – the Good Housekeeping of its day. Hale, who contributed to American folkways as the author of “Mary had a Little Lamb,” had been advocating in the magazine for a national day of Thanksgiving since 1837. Even as many states had begun to observe Thanksgiving, she wrote in 1860, “It will no longer be a partial and vacillating commemoration of our gratitude to our Heavenly Father, observed in one section or State, while other portions of our common country do not sympathize in the gratitude and gladness.”

So how did the lore of that Pilgrim repast get connected to Lincoln’s wartime proclamations?

The Plymouth “first Thanksgiving” dates from an October 1621 harvest celebration, an event at which the surviving passengers of the Mayflower – about half of the approximately 100 on board — were able to mark their communal harvest with a shared feast. By the account of the Pilgrim leader Edward Winslow, this event was no simple sit-down dinner, but a three-day revel. “Amongst other recreations,” Winslow wrote, “we exercised our arms, many of the Indians coming amongst us, and among the rest their greatest king Massasoit, with some ninety men, whom for three days we entertained and feasted, and they went out and killed five deer, which we brought to the plantation.”

There is nothing novel or uniquely American — and nothing especially “Pilgrim”– about giving thanks for a successful harvest. Certainly it has been done by people throughout history and surely by earlier Europeans in America as well as Native Americans.

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But New Englanders, who had long marked a Founders Day as a celebration of the Pilgrim and Puritan arrivals, began to move across America and took this tradition – and their singular version of history — with them. Essentially a churchgoing day with a meal that followed, the celebration of that legendary feast gradually evolved into the Thanksgiving we know.

Eventually, it was commingled with Lincoln’s first proclamation. During the post-Civil War period, the iconic Thanksgiving meal and the connection to the Pilgrims were cemented in the popular imagination, through artistic renderings of black-cloaked, churchgoing, gun-toting Puritans, a militant, faithful past that most likely rang familiar for many Civil War Americans.

But one crucial piece remained: The elevation of Thanksgiving to a true national holiday, a feat accomplished by Franklin D. Roosevelt. In 1939, with the nation still struggling out of the Great Depression, the traditional Thanksgiving Day fell on the last day of the month – a fifth Thursday. Worried retailers, for whom the holiday had already become the kickoff to the Christmas shopping season, feared this late date. Roosevelt agreed to move his holiday proclamation up one week to the fourth Thursday, thereby extending the critical shopping season.

Some states stuck to the traditional last Thursday date, and other Thanksgiving traditions, such as high school and college football championships, had already been scheduled. This led to Roosevelt critics deriding the earlier date as “Franksgiving.” With 32 states joining Roosevelt’s “Democratic Thanksgiving, ” 16 others stuck with the traditional date, or “Republican Thanksgiving.” After some congressional wrangling, in December 1941, Roosevelt signed the legislation making Thanksgiving a legal holiday on the fourth Thursday in November. And there it has remained.

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Kenneth C. Davis is the author of “Don’t Know Much About History” and “America’s Hidden History.” His forthcoming book, “The Hidden History of America At War: Untold Tales from Yorktown to Fallujah,” includes an account of the siege of Petersburg, Va.