The Land Rush of 1889

April 22, 1889:

At high noon, thousands rush to claim land in the Land Run of 1889. Within hours the cities of Oklahoma City and Guthrie are formed with populations of at least 10,000.
The Oklahoma Land Rush of 1889 was the first land run into the Unassigned Lands and included all or part of the 2005 modern day Canadian, Cleveland, Kingfisher, Logan, Oklahoma, and Payne counties of the U.S. state of Oklahoma. The land run started at high noon on April 22, 1889, with an estimated 50,000 people lined up for their piece of the available two million acres (8,000 km²).
The Unassigned Lands were considered some of the best unoccupied public land in the United States. The Indian Appropriations Bill of 1889 was passed and signed into law with an amendment by Illinois Representative William McKendree Springer, that authorized President Benjamin Harrison to open the two million acres (8,000 km²) for settlement. Due to the Homestead Act of 1862, signed by President Benjamin Harris, legal settlers could claim lots up to 160 acres (0.65 km²) in size. Provided a settler lived on the land and improved it, the settler could then receive the title to the land.A number of the individuals who participated in the run entered early and hid out until the legal time of entry to lay quick claim to some of the most choice homesteads. These people came to be identified as "sooners." This led to hundreds of legal contests that arose and were decided first at local land offices and eventually by the U.S. Department of the Interior. Arguments included what constituted the "legal time of entry." While some people think that the settlers who entered the territory at the legally appointed time were known as "boomers," the term actually refers to those who campaigned for the opening of the lands, led by David L. Payne.

The University of Oklahoma's fight song, "Boomer Sooner," derives from these two names.

What, more than anything made the United States a land of opportunity?  Available land in an arable region.  The land available in the Great Plains after the Homestead Act wasn't able to support agriculture like the land east of it which had been sold, but the readily available land throughout the nation was crucial for the development of a middle class.