The Pentagon will start to require active-duty troops to be vaccinated against COVID-19 in the middle of September, over the objections of some Republican members of Congress who say the policy is illegal and could spark legal challenges.

Previously, the Defense Department had followed its policy of not making a COVID-19 vaccine mandatory because the Food and Drug Administration had approved them under an emergency use authorization and not fully approved them after clinical trials.

But Defense Secretary Lloyd J. Austin III issued a memo Monday that requests a waiver from that policy from President Joe Biden, who immediately signaled his support in a written statement.

“Secretary Austin and I share an unshakable commitment to making sure our troops have every tool they need to do their jobs as safely as possible,” Biden said. “These vaccines will save lives.”

The Pentagon now has slightly more than a month to prepare to distribute and administer shots to troops who have up until now refused the vaccine.

A large portion of the more than 2 million active-duty and reserve servicemembers have been vaccinated already, with percentages varying between services: 74 percent of active-duty and reserves in the Navy, 65 percent active-duty and 60 percent of reservists in the Air Force, and the Army closer to 50 percent, according to The Associated Press, which first reported the memo.

If the FDA grants full approval to a version of the COVID-19 vaccine before the middle of September, the vaccine would become mandatory much sooner.

A number of fully approved vaccines for other diseases are already mandatory for members of the military, including anthrax, measles, mumps, smallpox and typhoid fever.

“Get the shot. Stay healthy. Stay ready,” Austin’s memo concludes.

House Armed Services Chairman Adam Smith, D-Wash., praised Austin’s decision, which he said would save lives and keep American forces ready to fight.

“Some may try and criticize the Secretary’s decision, using anti-vax arguments that are not supported by facts or science to politicize the conversation. These desperate attention seekers must be ignored,” Smith said in a news release. “The health and safety of our troops, and our national security, is what truly matters, and mandatory vaccination is the proven solution to provide protection from the COVID-19 virus and delta variant.”

In the House, 30 Republican members have co-sponsored legislation introduced by Rep. Thomas Massie, R-Ky., meant to block any requirement that military members get a COVID-19 vaccine. Four members of the House Armed Services Committee — Reps. Mo Brooks, R-Ala.; Scott DesJarlais, R-Tenn.; Matt Gaetz, R-Fla.; and Mark E. Green, R-Tenn. — are among the bill’s co-sponsors.

Green and 15 other members of Congress sent a letter to Austin last week opposing the requirement to be vaccinated without full authorization from the FDA.

Rep. Clay Higgins, R-La., sent a letter Monday that raises the threat of legal and “congressional action” that would stop Austin from requiring the vaccine.

“Further, the current reported plan to force vaccinations on military personnel prior to FDA approval will rightfully bring forth a strong Article III challenge on many fronts,” Higgins wrote.