The president of the United States has now, twice in the span of 18 months, unsubtly threatened genocidal violence against another country.

Let's not mince words here: Nuclear weapons are designed to inflict genocidal levels of destruction and death. They are a weapon to be used as a last resort, if that, to ensure national survival. They are not meant to be used casually — either as a weapon, or rhetorically. Not even implicitly.

But President Trump keeps doing exactly that.

The latest threat came late Sunday night, in an unhinged-sounding all-caps tweet that almost immediately ranks — along with Ronald Reagan's "we begin the bombing in five minutes" gaffe and JFK's Cuban Missile Crisis speech to the nation — as among the more alarming presidential communications in history.

Trump wrote: "To Iranian President Rouhani: NEVER, EVER THREATEN THE UNITED STATES AGAIN OR YOU WILL SUFFER CONSEQUENCES THE LIKES OF WHICH FEW THROUGHOUT HISTORY HAVE EVER SUFFERED BEFORE. WE ARE NO LONGER A COUNTRY THAT WILL STAND FOR YOUR DEMENTED WORDS OF VIOLENCE & DEATH. BE CAUTIOUS!"

This is utterly terrifying.

Rouhani's offending threat came in a speech on Sunday to Iran's diplomats, in which Rouhani warned America, essentially, not to meddle in Iran's affairs. "America should know that peace with Iran is the mother of all peace, and war with Iran is the mother of all wars," Rouhani said. It wasn't a pledge of nonviolence, certainly, but it wasn't an offensive threat, either. Instead, it was a warning of the kind that nations routinely make when they perceive threats gathering against them.

Trump's response was wildly dangerous.

First of all, Trump escalated. He didn't merely threaten Iran not to mess with the United States: He warned of "consequences the likes of which few throughout history have ever suffered." That's not an explicit reference to nuclear weapons, but it doesn't have to be: Given how few times nuclear weapons have been used in anger, that's precisely how Trump's threat will be received.

Trump also set the bar for U.S. action far too low.

Read his tweet again, in a more-silent voice: "We are no longer a country that will stand for your demented words of violence and death." Words. Trump is suggesting to the world that he's willing to go to war with Iran because Iran's leaders say hurtful or scary things. That is not even close to a good reason to go to war.

This isn't the first time Trump has done this, of course. Last year, he threatened "fire and fury" against North Korea, another explicitly implicit warning of nuclear war. That Trump now seems to be insisting he's achieved a peaceful solution to the crisis there should give us little consolation: At the moment he tweeted, it seemed to move America closer to a shattering war desired by few outside the Oval Office. So it would also seem with Trump's Sunday night tweet against Iran.

Again: Presidents don't — and shouldn't — make threats of nuclear war casually. Reagan's "bombing" joke was supposed to be off-the-record, not meant for public consumption, and it was treated as an international crisis. When Kennedy warned the Soviets against placing missiles in Cuba, signaling U.S. readiness to go to war, he still pleaded 10 different ways for peace.

"We have no wish to war with the Soviet Union — for we are a peaceful people who desire to live in peace with all other peoples," Kennedy said. He meant it: Peace prevailed. Trump, it seems likely, would consider such a statement a sign of weakness.

Trump's threat comes in a much larger context than the U.S. relationship with Iran: This tweet comes a week after his disastrous Helsinki press conference with Vladimir Putin, a week in which even members of his own party have warned against Trump's penchant for buttering up dictators while undermining American alliances. Simply put: He looks less like a protector of the United States than he did last month. It's impossible not to wonder if he's feeling extra motivation to act like a tough guy.

There's another context for Trump's threat: A growing sense that it's wrong for any president of the United States — be it Trump or Barack Obama, Democrat or Republican — to have unlimited command over the nation's nuclear weapons.

As is, Trump could, on his own authority, choose to start a nuclear genocide on a whim — and there's almost nothing in American law or the chain of command to stop him or even slow him down.

It's time for that to change.

Trump may be commander in chief of the armed forces, but it is Congress' responsibility under the Constitution to declare war. The legislative branch has largely shirked that responsibility in the post-World War II era, but Trump's tweets are one signal among many that it's time to re-engage.

Earlier this year, Sen. Ed Markey (D-Mass.) and Rep. Ted Lieu (D-Calif.) sponsored legislation requiring the president to receive congressional approval before initiating a first-use nuclear strike. "No one person should have the power to decide when the U.S. will be the first to use nuclear weapons," Markey declared on Twitter.

He's right. It's also true that no one person should have the power to threaten the use of U.S. nukes in willy nilly fashion, as President Trump has done. Nukes are too dangerous, the mere threat of their use has the potential to unleash instability throughout the world. Trump's threat doesn't make America safer, nor does it feel safer: Instead, his casual threats raise fears of a return to the bad old days of the Cold War, when the possibility of the end of the world loomed constantly in the background.

The best way to put a leash on Trump's wild threats is for Congress to assert its authority. The time to act, clearly, is now.