Old Spanish Trail
Part of the reason the Old Spanish Trail was used was because it linked via the Santa Fe Trail to Missouri and the United States, and via the the Camino Real to Chihuahua and Mexico City in Mexico. 
There were very few European settlements on this rugged route between Santa Fe and Los Angels except near each end of the trail.
Europeans did not tend to settle along this trade route in communities. However, because of the slave trade along trail the Indian tribes were cautious, hostile, and themselves engaged in slave trading and raids.
- Lifetime Legacies Productions and San Luis Valley Museum Association, History of the Old Spanish Trail
- National Park Service, Santa Fe Trail History, stories, and map
- NPS Santa Fe National Historic Trail Map fairly detailed map with historical notes
- Santa Fe Trail Research describes trail projects and research articles
- Wikipedia contributors, "Old Spanish Trail (trade route)" in Wikipedia: the Free Encyclopedia at http://en.wikipedia.org/wiki/Old_Spanish_Trail_(trade_route)(accessed 21 July 2009), and Lifetime Legacies Productions and San Luis Valley Museum Association, History of the Old Spanish Trail at http://museumtrail.org/OldSpanishTrail.asp (accessed 29 July 2009).
- Wikipedia contributors, "El Camino Real de Tierra Adentro" in Wikipedia: the Free Encyclopedia at http://en.wikipedia.org/wiki/El_Camino_Real_de_Tierra_Adentro (accessed 19 July 2009).