H. (possibly Henry) P. Heintzelman appears to have been born in Pennsylvania in about 1820, joined the United States Army, served in the Mexican War as part of the 1st Missouri Mounted Regiment, and also in California as an Indian Agent. Heintzelman was likewise a California State Senator during 1855-56. This biography only assumes that all of these duties were carried out by the same person, since some accounts spell the name as "Heintzleman." "Heintzelman" is what appears on the letter itself.
On its way to Chihuahua during the Mexican War, the 1st Missouri Mounted Regiment encountered a much larger Mexican Force on February 28, 1847, resulting in the Battle of the Sacramento. The Missouri Regiment defeated the Mexican force, captured all of their artillery, and marched unopposed into Chihuahua on March 2.
From the description of Mexican War letter describing the Battle of the Sacramento, 1847. (California State Library). WorldCat record id: 320913445
