President Trump has made it clear that in a second term, he’ll again push NATO allies to pay more on defense. Many imagine that as a threat to abandon the alliance, one opponents are acting to prevent — and thereby advancing Mr. Trump’s foreign policy goals even if he loses.

In last month’s bill funding the Department of Defense, senators included a safeguard for NATO. Signed into law by President Biden, it aims to allay fears at Washington and alliance capitals, but it also sent a message: Mr. Trump might call the bluff of Europeans who shout, “Yankee, go home.”

Only one candidate for the Republican nomination, Vivek Ramaswamy, has called for leaving NATO, as Politico reported Friday. Mr. Trump, though, criticized allies during his presidency for not meeting the two percent of GDP on defense spending agreed to by NATO defense ministers in 2006. He has also questioned the value of the alliance if members don’t share the cost of Europe’s defense.

In 2014, only three of NATO’s 30 non-American members met the two percent threshold. By 2020, after four years of Mr. Trump, the number had more than tripled to ten. “We expect this trend to continue,” NATO’s secretary-general, Jens Stoltenberg, told reporters at Stuttgart in October 2020.

Far from weakening the alliance, as critics feared, Mr. Trump’s campaign strengthened it, advancing toward a long-standing policy goal of Washington leaders in both parties. However, for those in soft-spoken diplomatic circles, the end didn’t justify the methods he employed.

So Senator Kaine and Senator Rubio, a Democrat and a Republican respectively,  co-sponsored the new provision for NATO. It requires an act of Congress or Senate approval to leave the alliance or be denied funding to bring the troops home.

The language is more posturing than practical. Congress would have to abandon American soldiers, sailors, and airmen in the crossfire of a Russian invasion. It would be akin to the infamous moment in 1975, as the Sun wrote in an editorial in September, when President Biden “was among three Democrats who refused to vote even for contingency funds to make sure Americans got out” of South Vietnam.

The latest limitation on NATO would also set up a legal clash. Article II of the Constitution states that the president alone “shall be commander-in-chief of the Army and Navy.” Another impractical option is attempting to force a president to prosecute a war he does not want, as war hawks in Congress considered when President McKinley held out for a diplomatic solution with Spain in 1898.

Furthermore, Article V of the NATO treaty itself offers a loophole. While summarized as “an attack on one is an attack on all,” Article V gives members wide flexibility in how they react to war. It states that in the event of a strike on a member, each treaty signatory agrees to take “such action as it deems necessary, including” but not requiring “the use of armed force.”

Members exercised this flexibility after 9/11, the only time Article V has been invoked. Britain provided a large military force in Afghanistan, but other nations did far less. Greece provided support ships. Turkey contributed only by allowing access to its airspace, although its role later expanded.

In the 1960s, France downsized its role in NATO, too. First, the French president, Charles De Gaulle, yanked his naval fleets from NATO command. Later, he announced that he intended to abrogate the treaty and demanded that all American troops leave France.

Mindful of the doughboys and GIs who had died “over there” for the liberté of France in the World Wars, President Lyndon Johnson ordered that his counterpart be asked if the expulsion included “the dead Americans in military cemeteries as well.”

De Gaulle’s brinkmanship arose from feeling — as Mr. Trump does — that his nation was being taken advantage of by other members. France remained in NATO but in a reduced role, not rejoining as a full partner until 2009.

 In “The Art of the Deal,” Mr. Trump described his “style of deal-making” in terms that mirrored De Gaulle’s. “I aim very high,” he wrote, “and then I just keep pushing and pushing and pushing to get what I’m after.”

Mr. Trump pushed NATO members to pay more and made that happen in his first term. In a second term, expect him to resume the pressure and to do so with an assist from Senate opponents, who just telegraphed that it’s wise for allies to shore up their own defenses to ensure the Yankees stay over there.