The disconnect between Trump’s attempt to signal resolve and the political backlash exposes a major weakness in Trump’s ability to talk tough in the international arena: his shaky political standing.

While running for office, Trump accused incumbent Barack Obama of coddling U.S. adversaries. He promised a tougher line against countries such as Iran and North Korea. But six months into Trump’s term, despite his condemnations of Iran and threats against North Korea, neither country seems cowed by him.

Trump’s strategy of making highly resolved statements about foreign policy may not be entirely wrongheaded. Research in my new book, “Statements of Resolve,” shows that the United States has tended to achieve better outcomes in international disputes when the president has made more resolute statements. But Trump’s domestic weaknesses might make him less able to convey resolve effectively compared with previous presidents.

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Resolute statements do often help U.S. presidents in foreign policy

Despite the common wisdom that talk is cheap, political science research shows that verbally conveying resolve can help the U.S. president succeed in foreign policy. Making public statements can put a president’s international reputation for credibility and possibly his domestic political support at risk, making it harder for the president to back down. Seeing a president boxed in by his statements can persuade adversaries of his resolve — and make them more likely to back down.

For example, after President John F. Kennedy made a resolute public speech during the Cuban missile crisis, the Soviet ambassador in Washington reported to Moscow that Kennedy “was staking his reputation as a statesman, and his chances for reelection in 1964, on the outcome of this crisis.” The belief that Kennedy was too committed to back down helped to persuade the Soviets to withdraw their missiles from Cuba.

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The effect of Kennedy’s resolute statements during the crisis was particularly dramatic because of the high stakes, but it was not necessarily unusual. In my research, I assign a numerical score for the level of resolve that the U.S. president conveyed with his statements in all post-World War II U.S. disputes. The United States has generally been more likely to win disputes in which the president conveyed more resolve.

But that depends on whether the president has the backing at home to follow through

But not all rhetoric is created equal. My research also shows that the effect of U.S. presidential statements of resolve is moderated by the president’s political ability to follow through. When the president is constrained by domestic politics, his statements of resolve are discounted — and, therefore, have less power to influence disputes.

The president may be commander in chief, but — as the world knows — he does not conduct foreign policy alone. He depends on advisers in his administration to present him with options and carry out instructions. He must have Congress’s backing, lest it constrain his foreign policy through budgetary powers, public opposition and statutory roadblocks.

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What’s more, a president seeking reelection depends on the public for support. Although launching military action can bump up public approval in the short term, known as a rally effect, it’s a risky strategy for presidents with shaky approval — because unsuccessful or high-casualty military actions can reduce approval over time.

U.S. adversaries know this. When they see a president is politically weak at home, they are more likely to disregard even the most resolute presidential statements.

The U.S. war in Vietnam is a classic example. During the war, President Lyndon B, Johnson lamented, “I so much wish that it were within my power to assure that all those in Hanoi could hear one simple message: America is committed to the defense of South Vietnam.”

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But the North Vietnamese heard more than one message. Yes, both Presidents Johnson and Richard M. Nixon resolved to fight as long as necessary. But even early in the war, the North Vietnamese Communist Party correctly predicted that U.S. public support for the war would weaken, when it declared, “Politics has always been the enemy’s [U.S.] weak point.” By the late 1960s, the public and members of Congress were increasingly vocal about their opposition to the war. That undermined efforts to display a resolute national front — and gave the North Vietnamese little incentive to back down.

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My broader quantitative analysis of presidential statements of resolve in U.S. disputes similarly finds that political weaknesses make it harder for presidents to convey credible resolve. For example, when more people disapprove of the president’s job performance than approve, there is no longer any notable relationship between resolute presidential statements and the probability of success.

Trump’s statements of resolve are not supported at home

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Trump’s attempt to convey firm resolve is undermined by his many domestic political problems and constraints. First, his approval rating has recently been at a historic low, and his biggest supporters are wary of foreign entanglements. Second, his relations with Congress are strained, creating greater congressional willingness to challenge the president in foreign affairs. Third, Trump’s own administration is full of divisions, and his own advisers have not always supported his preferences on issues such as the Iran nuclear deal. And in these latest statements on North Korea, Secretary of State Rex Tillerson appears to be moderating Trump’s rhetoric.

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If Trump tries to follow through with substantial military action, he’s likely to face pushback from the public, from his advisers and from Congress. To be sure, domestic troubles are not the only reason that Trump’s statements lack credibility. Trump has a reputation for speaking falsely and breaking pledges. And because his military options against North Korea are few and have many possible undesirable or even disastrous consequences, his threats are less credible.

Trump could increase U.S. credibility abroad by improving relations with others in government and rallying the public behind him. But such efforts would require sustained attention and would take time to pay off, and it’s not clear whether Trump would be willing to try.