When America first expanded west, one pioneer at a time, the Arkansas River – which bisects Pueblo – was the border between the United States and Old Mexico. Following the Mexican–American War (1846–48), developers began turning this eastern Colorado market town into a railroad hub and steel manufacturing center. It went on to become Colorado's second-largest city and eventually earned the moniker Pittsburgh of the West, thanks to the success of Colorado Fuel & Iron.
However, business flagged after WWII and with the steel market crash in 1982, Pueblo's steady decline turned into a free fall, which has only recently abated. But all that history makes for an interesting downtown wander. Seventy buildings and places are listed in the national historic registry, and plaques have been installed detailing local history along Grand and Union Aves, between 1st and B Sts.