Background: Plains Indians, Interior Plains Indians or Indigenous people of the Great Plains and Canadian Prairies are the Native American tribes and First Nation band governments who have traditionally lived on the greater Interior Plains (i.e. the Great Plains and the Canadian Prairies) in North America. Their historic nomadic culture and development of equestrian culture and resistance to domination by the government and military forces of Canada and the United States have made the Plains Indian culture groups an archetype in literature and art for American Indians everywhere. Plains Indians are usually divided into two broad classifications which overlap to some degree. The first group became a fully nomadic horse culture during the 18th and 19th centuries, following the vast herds of buffalo, although some tribes occasionally engaged in agriculture.
Context: The earliest Spanish explorers in the 16th century did not find the Plains Indians especially warlike. The Wichita in Kansas and Oklahoma lived in dispersed settlements with no defensive works. The Spanish initially had friendly contacts with the Apache (Querechos) in the Texas Panhandle.  Three factors led to a growing importance of warfare in Plains Indian culture. First, was the Spanish colonization of New Mexico which stimulated raids and counter-raids by Spaniards and Indians for goods and slaves. Second, was the contact of the Indians with French fur traders which increased rivalry among Indian tribes to control trade and trade routes. Third, was the acquisition of the horse and the greater mobility it afforded the Plains Indians. What evolved among the Plains Indians from the 17th to the late 19th century was warfare as both a means of livelihood and a sport.  The Plains Indians raided each other, the Spanish colonies, and, increasingly, the encroaching frontier of the Anglos for horses, and other property. They acquired guns and other European goods primarily by trade. Their principal trading products were buffalo hides and beaver pelts.  Although they could be tenacious in defense, Plains Indians warriors took the offensive mostly for material gain and individual prestige. The highest military honors were for "counting coup"--touching a live enemy. Battles between Indians often consisted of opposing warriors demonstrating their bravery rather than attempting to achieve concrete military objectives. The emphasis was on ambush and hit and run actions rather than closing with an enemy. Success was often counted by the number of horses or property obtained in the raid. Casualties were usually light. "Indians consider it foolhardiness to make an attack where it is certain some of them will be killed."  Due to their mobility, endurance, horsemanship, and knowledge of the vast plains that were their domain, the Plains Indians were often victors in their battles against the U.S. army in the American era from 1803 to about 1890. However, although Indians won many battles, they could not undertake lengthy campaigns. Indian armies could only be assembled for brief periods of time as warriors also had to hunt for food for their families. The exception to that was raids into Mexico by the Comanche and their allies in which the raiders often subsisted for months off the riches of Mexican haciendas and settlements. The basic weapon of the Indian warrior was the short, stout bow, designed for use on horseback and deadly, but only at short range. Guns were usually in short supply and ammunition scarce for Native warriors.
Question: what was the third factor?

Answer:
Third, was the acquisition of the horse and the greater mobility it afforded the Plains Indians.