Disunion follows the Civil War as it unfolded.

In the wake of South Carolina’s vote to secession in late December 1860, Americans both North and South anxiously wondered which state would be next to leave the Union. Little did they realize that the next call for secession would come not from a Southern state, but from a Northern city — New York City.

Library of Congress

On Jan. 7, 1861, two days before Mississippi became the second state to secede, Mayor Fernando Wood delivered a message to the Common Council, the city’s governing body, proposing that New York assert its independence as a “free city” by “disrupt[ing] the bands which bind her to a venal and corrupt master” — that is, the Union. Wood wanted the city, then comprised only of Manhattan Island, to become an independent city-state, akin to the seaport free cities of northern Germany. Indeed, he suggested that New York City’s founding charter — which established that “New York be, and from henceforth forever hereafter shall be and remain, a free city of itself” — already guaranteed its independence.

Wood’s call for secession was hardly surprising. A Democrat, he had been elected in 1859 on a vocal pro-Southern platform in a three-way election against a Tammany Hall Democrat and a Republican. What’s more, pro-Southern and pro-independence sentiment was widespread in New York, particularly among the merchant class. Their pro-independence stance was partly a matter of economic opportunism: New York was not only the richest and most populous city in the country, but it was also the critical source of federal tax revenue in these days before income taxes. In 1860, ad valorem taxes — tariffs on imported goods collected at ports — provided $56 million of the $64.6 million of federal revenue, and more than two-thirds of imports by value passed through New York. Secession would allow New York to keep that tariff revenue for itself, rather than pass it on to Washington. “As a free city,” Wood said, “with but nominal duty on imports, her local Government could be supported without taxation upon her people. Thus we could live free from taxes, and have cheap goods nearly duty free.”

Moreover, much of New York’s wealth came from its close ties to the South, a fact Wood emphasized in his message to the Common Council: “With our aggrieved brethren of the Slave States, we have friendly relations and a common sympathy.” Much of the South’s cotton exports passed through New York, and the city’s merchants took 40 cents of every dollar that Europeans paid for Southern cotton through warehouse fees, shipping, insurance and profits. Cotton — and hence slavery — helped build the new marble-fronted mercantile buildings in lower Manhattan, fill Broadway hotels and stores with customers, and build block after block of fashionable brownstones north of 14th Street. If seceding Southern states formed their own nation, New York merchants could expect to lose much of that lucrative trade. Southerners threatened to blacklist Northern companies they felt sided too closely with the Union and to unilaterally cancel debts owed to Northern merchants. New York’s elite — and the city’s economy — would be devastated.

Wood knew that his rhetoric against the North and the Republican Party — which he denounced together as “a people and a party that have plundered her revenues, attempted to ruin her” — would find ready popular support. The city was a bastion of anti-Lincoln sentiment, and many of New York’s leaders had played dirty to prevent Lincoln’s election: Wall Street tycoons staged a short-lived financial panic and then informed the press that it had been caused by fear of Lincoln’s victory. Newspapers whipped up anti-Lincoln fears among working-class and immigrant voters. James Gordon Bennett Sr., publisher of The New York Herald, the city’s largest newspaper, cautioned the city’s working classes that “if Lincoln is elected, you will have to compete with the labor of four million emancipated negroes.” The New York Daily News — edited by the mayor’s brother, Benjamin Wood — warned New Yorkers that if Lincoln won, “we shall find negroes among us thicker than blackberries swarming everywhere.”

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Following Lincoln’s election, many city leaders declared their sympathy for the South rather than the incoming Republican administration. On Dec. 15, 1860, 2,000 merchants met in an “Appeal to the South” — where even a nominally pro-Union speaker swore his loyalty to the “white race” should war come. In late January, a committee of city leaders from New York and Brooklyn delivered a petition to Washington with 40,000 signatures in favor of compromise with the South rather than war. And several New York financiers hit the hard-pressed federal government at one of its weakest spots, threatening to stop buying federal bonds. Horace Greeley, pro-Union publisher of The New York Tribune, promptly called the banks’ bluff by urging the Treasury to sell bonds directly to individuals without the intermediary of New York banks.

While business leaders tried to force the city government to boost the chances of an independent city and protect their livelihoods, some New Yorkers were ready to take bold vigilante actions in support of secession. Leading businessmen even hatched a plot — never carried out — to capture the government’s military property around the city, including ships, forts and the vast Brooklyn Navy Yard.

The secession of New York would have been a disastrous blow to Washington. If New York became a “free city of itself,” the federal government might have slid into bankruptcy — the Treasury was already near empty — and it would have faced nearly insurmountable odds in raising money and men to oppose the breakup of the now-splintering Union. It would have lost its largest port, the entrepot to thousands of miles of canals and railroads, the keystone in a grand arch of commerce connecting the American Great Lakes to European markets. And it would have opened the possibility that, in a war, the South would have military access to the heart of the Union.

But all dreams of Wood’s free and independent city collapsed when the first South Carolina cannon was fired at Fort Sumter on April 12, 1861. The tidal wave of support for the Union overwhelmed secessionist sentiment in the city. New York, alongside the rest of the North that April, proclaimed its loyalty to the United States. On April 20, hundreds of thousands of New Yorkers gathered in a massive patriotic rally in Union Square, by which time New York had already begun to provide vast amounts of money, men and supplies that would save the Union over the next four years.

The South, meanwhile, fumed. “New-York will be remembered with especial hatred by the South to the end of time,” raged the Richmond Whig on April 22. “Boston we have always known where to find; but this New-York, which has never turned against us till the hour of trial, and is now moving heaven and earth for our destruction.”

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John Lockwood and Charles Lockwood are the authors of the forthcoming book “The Siege of Washington: The Untold Story of the Twelve Days That Shook the Union.“