When retired American president John Adams wrote to a friend in December 1812 about the war then being waged along the Canada-U.S. border, he let loose with some boastful thoughts about how efficiently Canada could have been conquered if he was still at the helm of the young republic.

The hand-written claim from the second U.S. president that he'd have made "short work" of Canada during the War of 1812 — first by asserting naval control over the Great Lakes and then by amassing an unstoppable army of 35,000 soldiers — has emerged from a major U.S. collection of historic documents and is expected to sell for up to $70,000 at an auction next week in New York.

In the Dec. 29, 1812 letter to Benjamin Rush, a fellow signatory of the U.S. Declaration of Independence in 1776, Adams opines on "what course I would have pursued had I been continued President at this time."

Under Adams' direction, the War of 1812 might have, in fact, been remembered as the War of 1806. He confides to Rush that he "would have declared war against Great Britain five or six years ago, when the King issued that most atrocious of all violations of the Law of Nations, his proclamation for impressing seamen from our ships."...