Don’t make idle threats. It is the apex principle of foreign affairs. A credibility gap is dangerous in matters of war and peace.

What are we to make of President-elect Donald Trump’s flamboyant flirtations with invading and occupying the Panama Canal, seizing Greenland from Denmark, a NATO member, and making all hell break loose in the Middle East if Hamas rejects Trump’s ultimatum demanding the immediate release of hostages?

Is he serious? If so, NATO members would be required to come to Denmark’s defense against the United States and leave NATO in tatters and vulnerable to invasion by Russia. If so, the rule of law in international relations would be shredded and we would reenter a world where the strong do what they can and the weak suffer what they must.

Reoccupying the Panama Canal Zone would violate the Panama Canal Treaties aping Germany’s World War I scorn for the Belgian Neutrality Treaty as nothing but “a scrap of paper.” And with regard to the Middle East, there is not much more hell to inflict. Babies are freezing to death. Death certificates are issued before birth certificates. Starvation stalks Gaza.

In international affairs — especially in the nuclear age — credibility is nine-tenths of diplomacy. To repeatedly make threats but then back down is an earmark of weakness, not strength, which emboldens the enemy to adventurism. Trump is turning President Theodore Roosevelt’s “speak softly, but carry a big stick” on its head, something akin to “speak loudly carrying a twig hoping to bluff your opponent into capitulation.”

The president-elect is also backtracking on his threat to impose astronomical tariffs on imports from nations unable or unwilling to end the flow of undocumented immigrants and fentanyl notwithstanding the huge demand for the same in the United States.

He should learn from “The Boy Who Cried Wolf” in Aesop’s Fables. Foreign nations will come to disbelieve him if he repeatedly overplays his hand. Think of our tense and deteriorating relations with China over Taiwan. Suppose President Trump warns China to refrain from an invasion by threatening an overwhelming military response from the U.S. including nuclear weapons. Would China be deterred? Or would China call President Trump’s bluff and attack in the belief that his warning was nothing but bluster?

These unsettling questions should not even be on the table. International relations are not like bluffing your way to a successful business deal.