What event led to Oklahoma's nickname as "The Sooner State"?

Oklahoma earned the nickname "The Sooner State" primarily due to the events surrounding the Land Run of 1889. This event marked the opening of the Unassigned Lands in Oklahoma Territory, where settlers were allowed to claim plots of land. Many individuals, known as "Sooners," illegally entered the territory before the official start time of the land rush, thereby securing prime land for themselves. The term "sooner" became synonymous with these early settlers who sought to stake their claim ahead of the official start of the race for land. This significant historical moment in Oklahoma’s development not only led to its nickname but also played a pivotal role in its settlement and eventual statehood.

Other options represent significant historical events as well, but they do not connect directly to the origin of the nickname. For example, while the Civil War conflicts and the discovery of oil had important impacts on the state, they do not relate specifically to the reasons behind the term "Sooner." Similarly, although the opening of the first state fair is a notable achievement in Oklahoma's social history, it is not linked to the development of the nickname.