The battle between St. Louis and Chicago was not necessarily a zero-sum game. The Illinois city would have grown even if the Missouri entrepôt had continued to flourish. Well before David Atchison led his ruffians into Kansas, Chicago merchants began invading St. Louis's commercial hinterland. Moreover, some northeastern capitalists, such as John Murray Forbes, had become interested in Illinois railroads prior to the settlement of the new western territories. Although neither city had to collapse for the other to thrive, they competed for markets, migrants, and capital, and only one urban center could dominate the western economy.

The Kansas turmoil, however, transformed the development of both cities. Historians cannot know if Chicago would have supplanted St. Louis without the disruption spawned by Thayer, Atchison, and their followers; neither city's fate was inevitable. Nonetheless, the sectional controversy tipped an important balance in the Illinois city's favor. Between 1854 and 1856 political tensions sapped the commercial strength of St. Louis and dramatically accelerated the ascent of Chicago. Gradual shifts in investment, marketing, and migration patterns became abrupt changes, and the contest for economic control of the West became increasingly one-sided. New York and Boston capital poured into Illinois businesses; Chicago merchants gained control of important hinterland markets; and settlers made the lake city the new gateway to the West.