Here’s a potential surprise choice as President Trump’s national security adviser: Joe Biden. Mr. Trump would never offer it. Mr. Biden would never take it. The idea is worth reflection nonetheless, because on foreign policy, there isn’t that big a difference between the two. A public offer by Mr. Trump of the position to Mr. Biden would at least draw attention to Mr. Biden’s flip-flops over the years on foreign policy issues.

Sure, there have been differences between the Trump administration’s foreign policy and that of the Obama-Biden administration that preceded it.

The Obama-Biden administration signed a nuclear-deal with Iran that provided the regime in Tehran with $150 billion in sanctions relief. Mr. Trump canceled the deal and imposed economic pressure on Iran.

The Obama-Biden administration failed to move the American embassy in Israel to Jerusalem. Mr. Trump moved it.

The Obama-Biden administration loosened financial sanctions and travel restrictions on Cuba. Mr. Trump tightened them.

The Obama-Biden administration entered the Paris Climate Agreement on greenhouse gas emissions. Mr. Trump withdrew from it.

The Trump administration has tried to toughen enforcement of immigration laws. Mr. Biden has criticized that.

The Trump administration has tried to negotiate a nuclear deal with North Korea. Mr. Biden has criticized that.

Yet Mr. Biden was on the Trump side of all six of those issues before he was on the Obama side of them.

Iran sanctions? In the 2008 vice presidential debate against Governor Palin, Mr. Biden faulted the Bush administration for being too soft on Iran. “No one in the United States Senate has been a better friend to Israel than Joe Biden,” he said. “The only thing on the march is Iran. It’s closer to a bomb. Its proxies now have a major stake in Lebanon, as well as in the Gaza Strip with Hamas. We will change this policy with thoughtful, real, live diplomacy that understands that you must back Israel.”

Jerusalem? Senator Biden signed a March 20, 1995 letter to Secretary of State Christopher that said, “We believe that Jerusalem is and shall remain the undivided capital of the State of Israel . . . The search for peace can only be hindered by raising utterly unrealistic hopes about the future status of Jerusalem among the Palestinians and understandable fears among the Israeli population that their capital city may once again be divided by cinder block and barbed wire . . .

“The United States enjoys diplomatic relations with 184 countries. Of these, Israel is the only nation in which our embassy is not located in the functioning capital. This is an inappropriate message to friends in Israel and, more importantly, a dangerous message to Israel’s enemies. We believe that the United States Embassy belongs in Jerusalem.”

Cuba? A Reuters dispatch from 2009 is headlined “Biden says U.S. does not plan to lift Cuba embargo.” In 1996, Mr. Biden voted with Senator Helms and other hardliners to toughen sanctions on Cuba, against more liberal and farm state Democrats who wanted to sell American crops there.

Climate change? Climate Wire reports, “As Delaware’s senator, he spent years criticizing higher fuel efficiency standards. He voted at least five times against raising standards for tailpipe emissions . . . In 1991, when 36 Republicans and Democrats co-sponsored a bill to boost fuel efficiency standards, Biden declined to join. The year before, he had helped kill two proposals to lower ozone, particulate matter and greenhouse gas emissions from cars.”

Immigration? Here is Mr. Biden in a September 2007 Democratic presidential debate, asked about “sanctuary cities” that refuse to enforce federal immigration law. Moderator: “Senator Biden, yes or no, would you allow the cities to ignore the federal law?” Mr. Biden: “No.”

North Korea? Here is Senator Biden’s opening statement from a January 2004 Senate hearing on North Korea: “Three years ago, I urged the Bush Administration to test North Korea’s commitment to peace by putting a serious proposal on the table.

“No one knows if North Korea is prepared to abandon its pursuit of nuclear weapons. Frankly, it may prove impossible to convince North Korea to change path. But we won’t know until we try. So far, the administration has not made sufficient effort.”

Mr. Biden’s statement went on: “The outlines of a deal were clear three years ago, and they are clear today. North Korea must fully, irreversibly, and verifiably abandon its pursuit of nuclear weapons and its export of ballistic missiles. If North Korea commits itself to this path, the United States and its allies should stand ready to offer security assurances, sanctions relief, and normal diplomatic relations, matching action for action and words for words . . .

“Convincing North Korea to change course will not be easy. It requires a combination of sticks and carrots… we have frankly done a poor job of defining an alternative future for the North Korean people. We have had almost no dialogue with North Korea, holding just three meetings in three years, and all of them coming too late to do much good.”

So much of what passes for a foreign policy debate isn’t actually principled disagreement but just partisan political point-scoring. The party out of power bashes the party in power, and once the two sides switch, the policy positions switch. Both Messrs. Biden and Trump are constrained by American popular opinion, which oscillates between interventionism and isolationism as the costs of each become clear.

Neither one of them wants to be maneuvered into a costly long-term war, but neither one of them wants to put the American homeland or allies at risk or be humiliated by appealing impotent, either. Both of them are willing to negotiate with enemies under certain conditions. Both of them are guided by career diplomats and military officers and interest groups that remain influential in Washington regardless of which political party controls the White House.

Mr. Biden would be quite comfortable representing and implementing elements of the Trump foreign policy. In fact, he has plenty of experience at it already.

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Image: Drawing by Elliott Banfield, courtesy of the artist.