U.S. Army's in a foreign land of irregular partisans

The Mexican War (1846-1848) was the U.S. Army's first experience waging an extended conflict in a foreign land.
The Mexicans also employed irregular cavalry units, often raised from local ranchers and commanded by regular troops. In modern military terms, those forces would be designated as partisan fighters. "Partisan" describes organized guerrilla bands fighting under Mexican regular officers or officially sanctioned by the Mexican government. The term "partisan" did not enter the U.S. Army lexicon until 1863 in General Order No. 100, which differentiated between armed prowlers, guerrillas, and partisans. The term is appropriate in the current study, however, because there was considerable partisan activity during the conflict, especially in central Mexico.
They used a variety of tactics. The larger forces operated as cavalry units, which sought to engage quickly, to inflict maximum casualties, and then to disappear rapidly. The smaller units made extensive use of sharpshooters who concealed themselves in the trees and chaparral that lined the Mexican National Highway, which Scott's forces would use extensively. Imposing terrain features along most of the route's length worked to the advantage of the partisans.