By Bartee Haile

Speaking behind closed doors on Dec. 10, 1839, Mirabeau Lamar shared a startling secret with the Republic Congress. Texas’ second president informed legislators that for the past seven months he had been working to make peace with Mexico.

When Sam Houston passed the reins of government to his rival in December 1838, the new chief executive did not think a truce was in the cards. The Mexicans seemed dead-set on retaking the liberated province and punishing the San Jacinto vict...