Arkansas elected Republican Rep. Tom Cotton to the Senate on Tuesday, sending incumbent Mark Pryor packing. You might not be surprised that a Republican won in Arkansas in a heavily GOP year, but Cotton's victory is a much bigger deal than you probably think. His election is a significant step for the future of GOP foreign policy, and thus perhaps for American foreign policy. Cotton, you see, is the golden child of the Republican party's hawkish establishment. He still calls the 2003 Iraq invasion a "just and noble" war. He's young — just 37 — and fervently backed by some...