Platte Purchase.—The original western boundary of Missouri was a line drawn north and south through the mouth of the Kansas river. Soon after Missouri was admitted into the Union, however, the project of attaching to that state, what afterward became known as the 'Platte Purchase," was persistently urged by the citizens. The agitation began in 1835 (Col. William F. Switzler, in his "History of Missouri," p. 230, says Gen. Andrew S. Hughes started it in a speech which he delivered at a militia muster near Liberty, in Clay county, in that year). The territory included in the purchase lies between the Missouri river and the original state line. The idea of annexation met with immediate favor throughout the State of Missouri, and a memorial asking for it was sent to Congress in 1836. One difficulty in the way was that to comply with this request would make still larger a state which was already one of the largest in the Union, and a second difficulty was to remove Indians from a possession which had just been assigned to them in perpetuity.

Nevertheless, success came quickly. Senator Benton introduced a bill reciting that when the Indian title to that territory should be extinguished the jurisdiction over said tract should be "ceded to the State of Missouri." Benton's vigor, Senator Linn's adroitness and personal popularity, and the enthusiastic aid of Missouri's representatives in the other branch of Congress, did the work. No serious opposition was offered in either branch and President Jackson signed the bill on June 7, 1836. Missouri's legislature assented to the act on Dec. 16, the Sacs and Foxes had agreed to the terms for the relinquishment of their lands on Sept. 17, and on March 28, 1837, President Van Buren proclaimed the territory a part of the State of Missouri. Benton exultantly declared that the area of Missouri had thus been expanded "by an addition equal in extent to such states as Delaware and Rhode Island, and by its fertility equal to one of the third class of states."

The new territory, which is one of the richest parts of Missouri, comprises the counties of Andrew, Atchison, Buchanan, Holt, Nodaway and Platte. The "Platte purchase" also contributed St. Joseph, the third city in Missouri in population and wealth. Had it not been for this act of Congress, and had the western boundary of Missouri remained as stipulated in the organic act, the city of St. Joseph would today be in the State of Kansas, which would be a perfect parallelogram. But in 1836 the white man had not claimed dominion over the plains of Kansas and there was no one from this region to object to the annexation movement.