In 1845, Bliss took part in the United States military occupation of the Republic of Texas, prior to its annexation. Between April 1846 and November 1847, he took part in the Mexican War, including fighting in the battles of Palo Alto, Resaca de la Palma and Buena Vista. He was brevetted to Major in May 1846, and brevetted to Lieutenant Colonel in February 1847 for gallant and meritorious service.