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Mr. Burlingame said some abolitionists viewed colonization as a plot to preserve slavery by getting rid of free blacks in the North, while others saw it as a way to undermine slavery by fundamentally questioning the principles slavery was based on.

Mr. Magness, a researcher at the Institute for Humane Studies at George Mason University, said he first got wind of Lincoln’s efforts while researching a meeting between the 16th president and Union Gen. Benjamin Butler in the waning days of the war, at which colonization had been discussed.

Most of the U.S. documents about the Belize and Guyana deals have gone missing, but Mr. Magness and his co-author tracked down what he called an “almost untapped treasure cache of Civil War-era records” from the British side that showed Lincoln’s deep involvement in the planning and authorization.

With 4 million blacks in the U.S. at the time of the war, colonization would have been a tricky and pricey move.

The Belize project’s first shipment of laborers would have only been 500, and even if the project had been seen through to fruition, it would have accommodated just 50,000.