One can hear echoes of this Republican past in Mr. Trump’s own positions. His animating credo on foreign policy seems to be to farm out the heavy lifting to other countries whenever possible. Speaking on “The Hugh Hewitt Show” last August, he made his distaste for intervention clear: “At some point, we can’t be the policeman of the world. We have to rebuild our own country." Since then, to the consternation of the party establishment, he has also forthrightly denounced the Iraq war, declaring that the Bush administration’s case for it was based on a “lie.”

The Trump doctrine, if that term can be employed, is reminiscent of basic foreign policy realist tenets. In fact, as Thomas Wright of the Brookings Institution first pointed out in Politico, Mr. Trump has a “remarkably coherent and consistent worldview.” Mr. Trump, you could even say, is a spheres-of-influence kind of guy: Europe should take care of Ukraine, Russia should handle Syria. “When I see the policy of some of these people in our government,” he said on MSNBC this month, “we’ll be in the Middle East for another 15 years if we don’t end up losing by that time because our country is disintegrating.”

At the same time, he’s rejected the idea of repudiating the Obama administration’s Iran deal, and says that it’s important to remain “neutral” in the conflict between Israel and the Palestinians — two points that strike at the heart of Republican neocon orthodoxy. And he seems to have little use for alliances: He’s demanding that countries like Germany, Japan, South Korea and Saudi Arabia pay more for the United States to defend them. At the same time, he’s ready to slap high tariffs on Japan and China — something that could trigger a global depression.

Mr. Trump’s position can resemble realism on steroids. At bottom, he doesn’t want America to lead the world; he wants the world to get out of its way. Even many die-hard realists are unwilling to follow him: Last Friday his sinister advocacy of torture, which he has since disavowed, prompted not only neocons but prominent realists like Andrew J. Bacevich and Richard Betts to sign a letter called “Defending the Honor of the U.S. Military from Donald Trump” in Foreign Policy.

None of this seems to antagonize the Republican base, which appears less ideological on taxes and foreign policy than the party elite. Once George W. Bush and the neocons led us into Iraq, it was probably only a matter of time before the neocons were called to account. Maybe the surprising thing isn’t that the party is starting to morph back into its original incarnation, but that it took this long.