The arrival of 800 US National Guard troops in Washington, DC, at the direction of President Donald Trump has been framed as an "authoritarian push" by the mayor of the nation's capital.

Trump has also named other cities, including New York City, Los Angeles, Chicago and Baltimore, as possible future targets if they don't address their own local crime issues.

But it's not clear that Trump could replicate his capital intervention across state lines, and a court case in California, where he also deployed Guardsmen and Marines in June, could narrow his scope further. 

Trump cast his decision to involve the National Guard in Washington as a crime crackdown, calling it "Liberation Day" and claiming the city needed rescuing from "crime, bloodshed, bedlam and squalor."

The police will now sit under the supervision of US Attorney General Pam Bondi who has appointed Drug Enforcement Administration chief Terry Cole as "emergency police commissioner." 

But federal data contradicts Trump's claim, with statistics showing that DC's violent crime is at a 30-year low. Critics say that based on this, there is no emergency that requires a military presence in the capital.

"The numbers simply do not justify this measure," said Washington Mayor Muriel Bowser. 

So if crime has been going down in Washington, why the intervention? 

"It does look, if you look at the data, [like] crime is going down," said Laura Dickinson, a law professor at George Washington University, US. "City officials have not asked for help [from the president] so it really does seem at best questionable."

"This is really problematic and contrary to our tradition in the United States, where we've been very cautious about using the military to do law enforcement functions," Dickinson added.

Why is Donald Trump doing this?

In part, because he can.

Washington, DC (which stands for District of Columbia), is not part of any US state and largely falls under the jurisdiction of the federal government.

Under the 1973 Home Rule Act, presidents can take control of the DC police during emergencies for 30 days without congressional approval. And because it's a federal enclave, the president also commands the city's National Guard.

Some US commentators have observed the move could be a political attempt to distract from ongoing controversy related to the Epstein Files and the release of jobs figures that showed a rise in unemployment. Trump's popularity in his flagship policy areas — in particular immigration — has also recently declined. 

While Trump and his allies have pointed to city crime as justification for the move, his opponents inside and outside of congress say the action is designed to exact control over cities that do not support him.The Congressional Black Caucus, which currently has no Republican members, also said the cities named by Trump as potential targets all have the common thread of being led by Black mayors, labelling the move a "blatantly racist and despicable power grab."

Blue states and cities are opposing these moves, but can they oppose the president?

The presidency has fewer powers outside of the nation's capital. The governors of the 50 US states preside over the National Guard within their own borders.

It's a key distinction that William Banks, a law professor at Syracuse University, US, said would make it far more difficult for Trump to follow through on threats to extend interventions beyond the federal enclave of Washington, DC.

"It would be unwise, I think to generalize this example and apply it to other places in the United States," Banks told DW.

"He can't go to Chicago or Philadelphia or New York City or Los Angeles and do the same thing. He simply doesn't have the authority."

How could Trump send National Guard to Los Angeles?

But what about in June, when he deployed the National Guard and Marines to Los Angeles?

Banks said there are provisions for limited interventions to protect federal property and personnel, but Trump "was walking on very thin ice."

"His argument was that they were needed to ensure that the protesters didn't destroy federal property or harm ICE and other immigration personnel who are on the ground there doing their job."

While they can protect federal assets, military personnel are banned from being actively used in domestic policing by the Posse Comitatus Act. 

In California, a three-day trial investigating whether the deployment of the National Guard was in breach of this law, and potentially unconstitutional, has recently wrapped up, with a decision pending.

While the decision would only apply in California, it could give clarity on the outcome of future litigation if the president took a similar course in other states.

"Obviously, it would be quite relevant how the court decides," Dickinson told DW. 

But Dickinson believes there's a wider question confronting the armed forces — using them to patrol American cities could impact how these institutions are perceived by the public.

"It could damage the credibility and respect that Americans have for the military and the National Guard," Dickinson said. "These are some of the few institutions in the United States that enjoy very broad bipartisan support." 

What's likely to happen next?

Federal command of the DC police ends after 30 days, unless the Republican-controlled Congress approves an extension. The National Guard can remain active indefinitely.

Despite alarms being raised by Trump's opponents, who are calling the move an authoritarian flex, Banks expects a return to the status quo is more likely, particularly when it comes to threats to other states. 

He said the United States' foundational history overthrowing the British military, and the norm that law enforcement should be maintained by civilian police, are crucial in understanding what Americans will accept in their communities.

"Our situation is somewhat unique in the United States in not having any expectation of military involvement in law enforcement," Banks told DW. "We don't like military uniforms on our streets, we don't like men and women with guns patrolling our streets, it just rubs against the grain.

"Posse Comitatus codifies that principle, but I think the norm is even more important and more fundamental."

Edited by: Carla Bleiker