John Adams liked to point out that Thomas Jefferson came late to the revolution. Before arriving in Philadelphia in the spring of 1775, the Virginian “knew more of the Eclipses of Jupiters Satelites than he did of what was passing in Boston.” That wasn’t quite fair. But it’s true that Adams had been sowing the seeds of independence far longer than Jefferson, first in Boston in collaboration with his rabble-rousing cousin, Samuel Adams, and then in Philadelphia, where he’d served in the First Continental Congress in 1774. Jefferson, having skipped the First Congress entirely, showed up late to the second.

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He’d missed an eventful spring. In April of 1775, the first blood of the war had been spilled at Lexington and Concord.…