The Mier Expedition was an offshoot of the Somervell Expedition, an unsuccessful military operation launched in November 1842 by a Texian militia against Mexican border settlements. It included a major battle at Ciudad Mier on December 26 and 27, 1842 which ended with a costly Mexican victory. The attack was partly in hopes of financial gain and partly in retaliation for the Dawson Massacre (as named by Texans) earlier that year, in which thirty-six Texas militia were killed by the Mexican Army. Both conflicts were part of continuing efforts by each side to control the land between the Rio Grande and Nueces River. Texas believed that this territory had been ceded to the Republic in the Treaties of Velasco, by which they gained independence; but Mexico did not agree.