WASHINGTON — After President Donald Trump ordered more than 5,000 U.S. troops to the southwest border days before the midterm election to intercept what he called an “invasion” of migrants, retired Marine Col. David Lapan decided he could not stay silent.

“The idea that a group of poor people from Central America, most of whom are women and children, pose some kind of threat to the national security of the United States is ridiculous,” Lapan said in an interview. “It’s a misuse of active duty forces.”

Lapan held senior jobs at the Pentagon while in the Marines and then served in the Trump administration as a Department of Homeland Security political appointee before departing in late 2017.

He’s one of a growing number of former senior military officers who say Trump’s order to deploy troops to the border on the cusp of an election compromises the military’s traditional position as an institution shielded from electoral politics.

Trump has had rocky relations with the military since taking office, clashing with Pentagon leaders over his bar on transgender recruiting, his proposed space force, and his abrupt cancellation of training exercises in South Korea.

But Trump has added unusual strain by ordering a military operation whose timing and scale seem unjustified to some officers, and by suggesting military personnel might use deadly force against unarmed migrants, instead of remaining in a support role, as required by law.

Defense Secretary Jim Mattis has issued only terse press statements but has not said why a force of more than 5,000 troops — which Trump said could rise to as many as 15,000 — is needed to stop several thousand men, women and children who are heading north in hopes of applying for asylum at the U.S. border.

Asked Wednesday if the deployment on the eve of an election was a political stunt, Mattis replied: “We don’t do stunts in this department, thank you.”

But the torrent of criticism has included retired Army Gen. Martin Dempsey, the former chairman of the joint chiefs. He broke his near-total silence on Trump after the commander in chief suggested Thursday that U.S. troops might open fire on anyone who threw rocks at them along the border.

“A wasteful deployment of over-stretched Soldiers and Marines would be made much worse if they use force disproportional to the threat they face,” Dempsey tweeted Thursday. “They won’t,” he added.

The total price of Trump’s military deployment to the border, including the cost of National Guard forces that have been there since April, could climb well above $200 million by the end of 2018 and grow significantly if the deployments continue into next year, according to analyst estimates and Pentagon figures.

Trump pulled back on Friday, saying migrants who threw rocks would be arrested and prosecuted, not shot.

No active duty military personnel are known to have publicly criticized the border operation, though privately some say that opinions about the deployment within the military are divided, as among former service members.

Military personnel are instructed in training that they have a duty not to carry out illegal orders that violate the laws of war. If they have a moral objection to a policy decision, they are expected to resign from the armed forces.

Even Trump’s critics say he is within his legal power to order the operation.

“It’s always tough, especially if you are still in uniform,” Lapan said. “This isn’t an illegal order from anything I can see. Then it becomes much tougher. Is it politicization? Is it inappropriate?”

But the perception that one of Trump’s motives in sending troops to the border is to help Republicans in the election hurts the military’s status as an institution that by tradition has been insulated from electoral politics, some officers said.

“It’s politicization of one of the few remaining nonpolitical institutions in the country — the United States military,” Paul Yingling, a retired Army officer, said in an interview.

The official Pentagon orders given to units deploying to the border describe a dire situation.

“The security of the United States is imperiled by a drastic surge of illegal drugs, dangerous gang activity and extensive illegal immigration,” it reads.

But military planners anticipate that only a small percentage of the migrants will reach the U.S. border.

According to military planning documents, about 20 percent of the roughly 7,000 migrants traveling through Mexico are likely to complete the journey. The unclassified report was published by Newsweek on Thursday. If the military’s assessment is accurate, it would mean the United States is positioning five soldiers on the border for every one caravan member expected to arrive there.

Trump has depicted the caravans as a grave danger to U.S. national security, claiming they are composed of “unknown Middle Easterners,” hardened criminals and “very tough fighters.”

But the report, dated Oct. 27, notes that caravan members are unlikely to arrive for at least two to four weeks. Among those traveling are “limited #s of Bangladeshi, Haitian and African individuals,” it reads. It makes no mention of Middle Easterners.

The Washington Post contributed.