President Trump alone knows the codes that can authorize a nuclear attack on North Korea. Lately, it sounds like he hasn't ruled that out. This week alone, he's threatened North Korea with “fire and fury” and “things that will happen to them like they never thought possible,” and Friday morning, he tweeted this:

Congress could wiggle itself into the debate to stop Trump from launching nuclear warheads at North Korea. At least one bill to that effect has been introduced in the Senate.

Except, it might not want to. There are some major downsides to putting in restrictions on the president's use of nuclear weapons, including that doing so could make it easier for a country to attack the United States.

Let's start with the basics

The president cannot declare war without Congress's approval, and to many, the use of a nuclear weapon is an act of war. But it isn't that clear-cut.

“When it comes to Congress's ability to intervene, it's exactly the same as it would be for any other situation on proposed use of force,” said Rebecca Hersman, who served as deputy assistant secretary of defense for countering weapons of mass destruction in the Obama administration and is now with the Center for Strategic and International Studies.

Sen. Edward J. Markey (D-Mass.) and Rep. Ted Lieu (D-Calif.) introduced legislation in January that makes it doubly clear the president can't fire nuclear weapons pre-emptively without Congress's approval.

“This goes to the role which the Congress constitutionally should play in the declaration of war,” Markey said on MSNBC on Thursday. “We've allowed for the power with regard to conventional war to slip away from us, unfortunately.”

But there's a lot of gray area to the War Powers Act that gives the president the legal space to launch weapons on his own, nuclear or conventional.

As I explained after Trump unilaterally bombed Syria in April, the War Powers Act allows the president to take some military action on his own, as long as it's more of a one-off thing and not long-term.

The line on when a president needs Congress's approval is fuzzy, but it's more of a know-it-when-you-see-it-situation, said Phillip Carter, a senior fellow with the national security-focused Center for a New American Security think tank. A president's unilateral power, he said, is “something short of war. It's the use of force by the president to achieve an immediate end,” he said.

Technically, nuclear war isn't different under the law. If Trump wants to start something sustained, he will need to get Congress's buy-in.

But nuclear war is also MUCH different than other wars

Here are four important things we know about North Korea's quest for nuclear weapons. (Elyse Samuels/The Washington Post)

The whole reason the president can launch a nuclear strike without Congress's approval is to let other nuclear war powers know that the United States means business.

It's a deterrent to North Korea's Kim Jong Un and the like. If they fire nuclear warheads at the United States (North Korea is getting closer to being able to do that sooner than experts had predicted), they know that in a matter of minutes, U.S. nuclear warheads could be soaring over to them.

If Congress trims the president's immediate launch-strike power, it could be very, very, dangerous for the United States, Hersman said.

"[Nuclear weapons] exist to protect us from catastrophe. Their role in the world is to prevent their use and to deter their use,” Hersman said. “You need the president to be able to react responsibly and quickly.”

The dividing line here seems to be a preemptive attack. Congress is probably on okay legal ground to chime in before Trump launches something on his own. (He's left open that possibility by never once specifying what would cause him to pull the trigger on the U.S.'s locked-and-loaded arsenal.)

And Congress seems to understand that.

“In the event of a nuclear attack against the United States it would be different,” Markey said on MSNBC. " … with regard to the use of nuclear weapons where we have not been attacked with nuclear weapons … it would just be catastrophic for the planet, and that's why I believe we need constraints on the use of nuclear weapons.”

Congress doesn't have the political will to act right now

Markey and Lieu are clearly hoping this war of words between Trump and North Korea will give their bill momentum, something it seriously lacks. With Republicans in control of Congress, the bill has sat, gathering dust, since they introduced it in January.

Republican leaders in Congress have been reluctant to confront Trump, except when it comes to Russia. They recently forced Trump to sign a Russia sanctions bill, and some GOP lawmakers have introduced legislation to curb Trump's power to fire the special counsel investigating possible Russian interference in the 2016 election.

But reinserting themselves in a power that is traditionally centered on the executive branch would be a bold political move, and there's no evidence they want to.

Also: Why put restrictions on Trump, and no other president to date?

There's one more national security dilemma stirred up by putting preemptive protections on the president. If Congress does jump in, it's worth asking why they are doing it for THIS president. Could it be because they don't trust him, personally?

At least 61 percent of Americans say they are “uneasy” about Trump's ability to handle North Korea, according to a new CBS poll. And Trump mentioned in a debate in September that he wouldn't take a preemptive nuclear strike off the table.

Still, it's very troubling to insert politics to tinker with America's nuclear deterrence system, Hersman warned.

“The day we get in the business of designing our national security policies by personality, in permanent ways, that is a day we really step backward in terms of protecting Americans and their national security,” she said.

So, Congress could stop Trump, if it wants to. But it may not want to.