Frustrated with lagging military vaccinations, an Alabama base requires some G.I.s to provide proof of shots.

About 68 percent of members of the U.S. military have had at least one dose of a Covid vaccine, but given that only full vaccination affords significant protection against the more infectious Delta variant surging in parts of the United States, American commanders are seeking new ways to pressure, entice and cajole service members to get their shots.

Now, Fort Rucker in Alabama has become the first military base in the United States to require that unmasked uniformed service members provide proof of vaccinations.

As in other states that have low overall vaccination rates, coronavirus cases are rising sharply in Alabama. Only 33 percent of the state’s population is fully vaccinated, one of the lowest proportions in the country, and cases have shot up 133 percent over the last two weeks, according to a New York Times database, reaching a daily average of more than 500. Hospitalizations have risen 39 percent over two weeks.

“The big difference is going to be that if you are not wearing a mask, the leadership will be able to ask you, ask soldiers, to prove that they’ve been vaccinated by showing their vaccination card,” said Major Gen. David Francis, the commanding general of Fort Rucker, in a video posted July 12 on Fort Rucker’s Facebook page.

President Biden could legally require members of the military to get vaccinated even though the vaccines in use in the United States — those made by Pfizer-BioNTech, Moderna and Johnson & Johnson — have only emergency authorization from the Food and Drug Administration. Mr. Biden has steadfastly refused to exercise that power even amid the spread of the Delta variant.

And some military leaders insist that the lack of full F.D.A. approval prevents them from requiring that Covid shots join myriad other compulsory vaccines for service members.

There are about 5,000 uniformed personnel assigned to Fort Rucker. Many larger bases, like Fort Hood in Texas, require all service members to wear masks indoors, but have areas, like specified gyms, where those who have been vaccinated may congregate without masks.

More than 80 percent of active-duty service members are under 35, a group that has led resistance among civilians too. The vaccination rates vary by service branch. For example, 77 percent of active-duty members in the Navy have had at least one shot, Pentagon officials said recently, while in the Marine Corps, that number is 58 percent.