When the U.S. Census Bureau announced the closing of the frontier in 1890, American foreign policy made a fateful shift. Politicians, intellectuals and industrialists began to wonder what the guiding force in expansion would be. The frontier was significant because it allowed our national character to shift from being European to being American. We had to invent new ways of solving problems, and these solutions began changing our national character into a rugged, individualistic and liberty-loving people.
Frederick Jackson Turner wrote that the closing of the frontier meant that the motor for American change had to shift, and many business magnates in Washington began casting an eye towards the Pacific. Trade opportunities, territorial acquisitions and the potential for growth all lay there.
So, the most important result of the closing of the frontier was that American began moving towards becoming an imperialist nation.