Defense Secretary Lloyd Austin will add the COVID-19 vaccine to its list of mandatory shots by mid-September, requiring them for the roughly two million service members and setting up clashes with conservative lawmakers and some rank and file service members.

“There are going to be court cases…. It’ll be a mess,” Mark Cancian, a senior adviser at the Center for Strategic and International Studies, told The Hill. “I think it’s going to engender a lot of unnecessary backlash.”

Some lawmakers are already threatening to intervene with congressional action.

“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,” Rep. Clay Higgins (R-La.) wrote in a letter to Austin.

DOD originally planned to keep the shot voluntary until the Food and Drug Administration gives full authorization. The vaccines are currently authorized only for emergency use, with full authorization in the works.

But President Joe Biden directed the Pentagon earlier this month to determine “how and when” the vaccine will be added to the list of other required inoculations.

“Members of the military understand when you sign up for the military, that there are requirements laid upon you,” Pentagon Press Secretary John Kirby told reporters last week. “Some of those requirements include being healthy and fit and ready to serve.”

Almost 75% of active duty service members have received at least one shot of the COVID-19 vaccine.

Army photo by Sgt. 1st Class Gary A. Witte