The United States was willing to pay more per mile for the Gadsden Purchase than for the Mexican Cession because the Gadsden Purchase gave money to the United States to build a railroad.
United States fought against Spain in 1812.
So, your answer is Spain
Hope this helps! :)
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Tensions had been brewing throughout Europe—especially in the troubled Balkan region of southeast Europe—for years before World War I actually broke out.