My theory of presidential behavior is based on two previous findings from political science. First, from Richard Neustadt and Stephen Skowronek to Terry Moe and William Howell, scholars have shown that presidents have incentives to exercise and expand the powers at their disposal. Since foreign policy is an area where presidents face few constraints, they are especially prone to intervene with military force abroad — regardless of their previous campaign rhetoric or party ideology.

Second, as Frances Lee has shown, parties see politics as a zero-sum game. I show that when they control the White House, they often justify their own president’s interventionist behavior — but when they are in opposition, they criticize foreign intervention.

Thus, in a presidential campaign — all other things being equal — the challenger’s party is usually less interventionist than the incumbent’s party. But if the challenger wins, he or she will probably pursue a foreign policy as interventionist as that of the predecessor they criticized in the campaign. At that point, the two parties will change their positions on foreign policy accordingly.

To test this theory of presidential and partisan behavior, I examined U.S. presidents’ foreign policy actions and the two major parties’ ideologies from 1900 to 2016. Almost every president pursued an interventionist foreign policy regardless of previous ideological commitments, and in almost every instance the two parties changed their views about foreign intervention as predicted

For example, at the turn of the 20th century, Republicans in control of the presidency were significantly more internationalist and hawkish than their Democratic counterparts. They nominated candidates like Teddy Roosevelt, who called for the U.S. to “carry a big stick” and exercise “an international police power.” Democrats, on the other hand, criticized Republican “militarism” and “imperialism,” and nominated candidates like the isolationist populist William Jennings Bryan.