Washington — Shaun Byrnes and Jon Gundersen have served multiple presidents throughout their decades in the military and State Department. Now, as retirees, they're taking on the commander-in-chief in a court of law. 

In February, Gundersen and Byrnes, alongside another Vietnam veteran and a historian, sued to stop construction of the 250-foot arch that is set to be built in a currently empty traffic circle between Arlington National Cemetery and the Lincoln Memorial.

They argue the project has been rushed and the administration hasn't gotten proper congressional approval. The arch, they say, would disrupt the symbolic connection between the Lincoln Memorial and the Robert E. Lee Memorial, a carefully considered sightline meant to convey unity after the Civil War.

According to recent renderings, the arch would be more than double the height of the Lincoln Memorial.

Gundersen and Byrnes say their lawsuit challenging President Trump's triumphal arch at the entrance to Arlington Cemetery isn't a partisan quest. 

"I think what we're doing is being loyal to the country. And loyalty can be measured in different ways," Gundersen, a retired Army Special Forces officer, told CBS News in an interview. 

Byrnes said he joined the suit, which is led by the Public Citizen Litigation Group, out of respect for fallen soldiers buried in Arlington.

"It's more about the duty I feel towards my colleagues and friends who did not come home to stand up against this project, regardless of who's in charge," Byrnes, a Navy veteran who served two tours in Vietnam, said. "I think it's just disrespectful to those that I served with who didn't come back, and then, of course, to all those who are lying in Arlington National Cemetery."

Byrnes had hoped to be interred in Arlington National Cemetery, but if the arch is built, he said he would "reconsider" his burial location.

The Justice Department has moved to dismiss the lawsuit, claiming the group lacks proper standing to sue. While no formal vote has been held by the current Congress, administration officials have suggested Congress already authorized the project, pointing to a plan adopted more than 100 years ago. It called for the construction of "two stately columns," 166 feet tall, that symbolize the North and South.

Last month, a federal judge denied a motion to temporarily halt the construction, which hasn't begun yet.

Gundersen, 81, and Byrnes, 83, view the arch not as a commemoration of America's 250th birthday but as a monument to one man: Mr. Trump. 

Last year, Mr. Trump showed off a model of the development project, which resembles the Parisian Arc de Triomphe, telling a group of reporters in the Oval Office that the structure is "going to be fantastic."

When asked by CBS News' Ed O'Keefe who the arch is for, Mr. Trump pointed to himself. 

"Me," he said. 

In a key step in moving the project forward, last week the U.S. Commission for Fine Arts approved the arch, despite overwhelming public opposition.

At the CFA meeting last week, the group's secretary, Thomas Luebke, said that "100% of the comments were against the project," The Associated Press reported. An urban planning commission, also stocked with allies of the president, could sign off on the project in early June.

Gundersen and Byrnes, who were both dispatched to Moscow in their careers and helped open U.S. embassies in post-Soviet Eastern Bloc countries, said the so-called "Arc de Trump" reminds them of countries they have fought and worked in.

"We know how authoritarian dictatorships work," Gundersen said. "There's no rule of law, there's no consent of the governed, and there's monuments for the leaders there."

Gundersen also found little reassurance in Mr. Trump's suggestion that the arch would be paid for by private donations, not taxpayer dollars, saying, "Even if you took private donations, is that how we want to build monuments? To the oligarchs who give money for favors?"

"We have fought for our country. We believe in this country, and we're going to continue to the end, and I think we can change things," he said.