We can’t go on like this. Credibility is essential to the effective and safe conduct of national security. No amount of hasty cleanup will erase the words that come from the lips of a commander in chief. And no, it is not a defense of the president to note—accurately—that his immediate predecessor was as notorious for his verbal indiscipline as Mr. Biden is.

For now, we have an immediate and escalating problem with this presidency. We can certainly hope that Russians understand as well as we do that, at 79, Mr. Biden is prone to saying things he doesn’t mean. But we can’t be sure. What we can be sure of is that Mr. Putin, who has already whipped up his compatriots into a frenzy of paranoia about the “real” intentions of the U.S. in arming Ukraine—to wit, an attempt to weaken and destroy Russia itself—will seize on every piece of evidence he can find to bolster his case.

Diplomacy is a subtle activity that combines artful deception with necessary candor. States convey to each other only what they want or need to convey; they willfully mislead each other about some aspects of their objectives and capabilities while drawing bright red lines around their nonnegotiable truths. Strategic ambiguity helps induce in allies and adversaries alike a distinct uncertainty about intentions. But clarity is essential when the stakes are existential. Decoding these complex messages, sifting the signal from the noise, is the essence of successful statecraft.