In 1821 William Becknell and five comrades traveled from Franklin, Missouri to Santa Fe, New Mexico, then the northern provincial capital of New Spain, the first Americans to do so legally. And thus was born the Santa Fe Trail, a nine hundred mile long road of commerce to a foreign land. During New Spain’s reign, foreign trade had been forbidden, but that changed when Mexico wrested control from the European empire in 1821. Never an active immigrant highway, selling merchandise to goods-starved...