Long fascinated with military pomp, Trump has openly envied the military spectaculars seen in cities from Paris to Moscow and Pyongyang ever since his first term as president.

The $45 million parade is officially being held to celebrate the 250th anniversary of the US army, which commander-in-chief Trump this week called the "greatest fighting force ever to walk the face of the Earth."

But critics say the parade is more about Trump than the army.

Protesters have pledged to rally on Saturday against what they call Trump's growing authoritarianism, at a time when he just ordered troops into Los Angeles after demonstrations against his immigration policy.

So-called "No Kings" rallies -- named after the idea that America's Revolutionary War against the British was to free the country from monarchs and autocrats -- are planned in dozens of cities, including just outside Washington.

But Trump is unrepentant.

The president said on a visit to the Fort Bragg army base on Tuesday that "we want to show off a little bit" with the parade, and vowed "very big force" if protesters try to disrupt it.

He made the comments in an extraordinary speech that breached the usual separation of politics and the military and saw Trump goad troops into jeering his opponents.

- 'Big birthday party' -

Trump's long-cherished parade plans are also rare for a country which has traditionally preferred to avoid displays of military might on its own soil.

The parade will be the biggest in Washington since 1991 after the first Gulf War -- and before that for the inauguration of President Dwight D. Eisenhower in 1949, the army said.

Nearly 7,000 army soldiers will march past historic landmarks including the Lincoln Memorial, the Washington Monument and the White House.

Roaring overhead will be a fleet of more than 50 helicopters including Apache gunships, giant twin-rotor Chinook transport choppers and sleek Black Hawks.

Around 150 military vehicles -- including 28 M1A1 Abrams battle tanks, 28 Bradley armored vehicles and 28 Stryker vehicles -- will rumble along the route.

Following the parade, the army's Golden Knights parachute team will jump in and present Trump with a US flag.

Troops have been moving tanks and other hardware into place throughout the week.

"I think the reception here is going to be very warm," US army Colonel Kamil Sztalkoper told AFP during a media preview. "Who doesn't like a big birthday party when you're 250 years old?"

- 'Believe in democracy' -

But the display of American muscle is also a flex of Trump's own strongman image as commander-in-chief, at the start of a second term when he has been pushing US presidential power further than ever before.

Trump has been obsessed with having a parade since his first term as president when he attended France's annual Bastille Day parade in Paris at the invitation of Emmanuel Macron in 2017.

Back then he was put off by the huge cost, then estimated at $92 million, and warnings that heavy tanks could damage Washington's streets.

This time, the army says metal plates will protect the roads.

At the time it also sparked comparisons to similar events in autocratic countries like Russia, China and North Korea -- comparisons which have resurfaced in his second term.

Peter Loge, director of George Washington University's School of Media, said the American aversion to such displays went back to the earliest days of US independence.

"We were founded by a group of merchants and farmers who were tired of a standing army invading their streets in the name of keeping them safe," Loge told AFP.

"We've always looked down on grand military parades in Russia across Red Square or in North Korea, because we're not like that. We're Americans, and we believe in democracy, not in military shows of force."

Trump's show of US military might does however come at a time of mounting international tensions.

Fears of a Middle East conflict are on the rise as talks on Iran's nuclear talks wobble and Israel threatens to strike its facilities.