The effect that Fredrick Jackson Turner's theory had on American foreign policy was that The United States promoted imperialism in order to expand its influence.
Why did the United States become imperialistic?
In the 1890s, the United States had expanded to its furthest extent on the American mainland, according to Fredrick Jackson Turner.
The United States then turned to nations outside the mainland and promoted imperialism to get control of those areas. This is how the United States ended up with Hawaii and the Philippines.
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