The District of Columbia and Virginia filed a joint lawsuit on Saturday to halt construction of the triumphal arch President Donald Trump wants built near Arlington National Cemetery, calling the project "flagrantly unlawful" and "legally indefensible".

The complaint asks a federal court to bar construction before it starts. It warns that once work is under way, the administration could argue that pausing it would pose a national security risk, a tactic the lawsuit says has been used before on other contested projects.

"This is not the first time that Defendants have rushed to dramatically remake the District too quickly for courts to intervene," the lawsuit says. "This Court should not allow this effort to escape judicial scrutiny."

What is being built

The arch would rise 250ft (76 metres) on a patch of grass off the bridge that spans the Potomac River, connecting Washington with Arlington National Cemetery. At that height it would be about 100ft taller than the Statue of Liberty from its base to the top of the torch.

Trump has championed the project as a monument to the country's 250th anniversary. Local governments and preservation groups have opposed it on grounds of traffic disruption, historic views and the process used to approve it.

The administration says construction could begin as early as next week, which is why the lawsuit was filed now rather than after permits were finalised.

What local governments argue

The monument has drawn fierce opposition from local governments that fear disruption to traffic and historic views, and the lawsuit says the federal government has moved too fast for courts to intervene. The complaint asks for a ruling before construction begins, arguing that once work starts the government can frame any pause as a security risk.

Three lawsuits, one judge

Saturday's complaint brings the total number of lawsuits against the arch to three. A separate case brought by three Vietnam war veterans and an architectural historian is already before US District Judge Tanya Chutkan, who held a hearing on Friday on whether to halt the project pending resolution of that case.

Chutkan did not act immediately but said she would move expeditiously. The new lawsuit is intended to add pressure before any construction begins, and it raises the stakes for the judge's decision.

The two attorneys general behind the joint suit are Brian Schwalb of the District of Columbia and Jay Jones of Virginia. Their argument is that the federal government cannot override local and state interests in the site without following the required legal process.

What the government says

The administration says construction could begin as early as next week, which is the fact that drives the urgency of the new complaint. The government has not said it will pause while the courts consider the challenges, and that is precisely what the plaintiffs want a judge to order before any work starts.

Why the timing is the point

The legal fight is as much about sequencing as about the merits. Once construction starts, courts are more reluctant to issue injunctions, and the government can argue that stopping work would waste public money or create security risks.

That is why the lawsuit asks for relief before the first shovel. It is also why the administration's statement that work could begin next week matters: it sets a deadline for the court.

The height comparison

At 250ft, the arch would stand about 100ft taller than the Statue of Liberty measured from its base to the top of the torch. The site is a patch of grass off the bridge over the Potomac, between the capital and Arlington National Cemetery, which is why both the district and the state say they have a direct interest in what is built there.

The wider pattern

The arch is one of several projects in which the administration has moved quickly on symbolic construction while local officials say they were not consulted. The pattern has produced litigation in Washington and in other cities, and it has put federal judges in the position of deciding how fast the government can build.

For Trump, the monument is a legacy project with a clear political message. For the local governments, it is a test of whether federal power can be exercised over the capital and its neighbours without their consent.

What the judge can do

Judge Chutkan can issue a temporary restraining order, a preliminary injunction, or nothing at all. Each has different legal consequences. A temporary order would stop work almost immediately; an injunction would hold until the case is resolved; doing nothing would let construction proceed while the litigation continues. The plaintiffs are asking for the first two, and the government is arguing that neither is warranted.

What to watch

Three things will determine what happens next. First, whether Judge Chutkan issues a temporary halt before construction begins. Second, whether the government tries to start work while the cases are pending. Third, whether the three lawsuits are consolidated into one proceeding, which would speed up a decision.

For readers following the legal and political threads, our coverage of the Supreme Court's 9/11 plea denial shows how federal courts handle emergency applications, the California election officials ruling is another case of a judge checking the administration, and the Obama-Trump arts rally captures the political symbolism around such projects.

The confirmed facts are the lawsuit, the three cases, the height of the proposed arch and the administration's timetable. Whether construction is stopped is up to the courts.