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Answers:
Main Reason:
The French and Indian War began over the specific issue of whether the upper Ohio River valley was a part of the British Empire, and therefore open for trade and settlement by Virginians and Pennsylvanians, or part of the French Empire.
By the time of the California Gold Rush in 1849, overland trails had already been blazed westward and after the Civil War, the Homestead Act was passed in 1862, which sent thousands more settlers westward. During these years, Native American tribes were made to move from their lands, causing a number of Indian wars.