Mesilla (māsē`yä), town (1990 pop. 1,975), SW N.Mex., on the Rio Grande and near Las Cruces; settled c.1850. The whole Mesilla Valley became part of the United States under the Gadsden Purchase (1853). Mesilla was a central station on the overland mail route. From July, 1861, to Aug., 1862, it was headquarters for Col. John R. Baylor of the Confederate army, who proclaimed Mesilla the capital of the new Confederate territory. A museum commemorates Billy the Kid, who once stood trial there.