In 1848, after almost two years of fighting, the US annexation of former Mexican territory was signed into a treaty.
Mexico lost a third of its territory, land which would later yield metals and stretch the United States from coast to coast.
To find out how the war broke out, and how it ended in the deal that it did, Don spoke to Peter Guardino from Indiana University. Peter's third book, The Dead March: A History of the Mexican-American War, is a social and cultural history of this 1846-48 war.
Produced by Sophie Gee. Edited by Siobhan Dale. Senior Producer was Charlotte Long.