Like thousands of other soldiers in Angola, and thousands more across Africa, Sergeant Leiria has H.I.V., the virus that causes AIDS.

Struggling to stay fit for duty, he is fighting a battle he does not expect to win. But the war is one that the armed forces of Africa, already the epicenter of the epidemic, cannot afford to lose.

For better or worse, no institution is more central to the stability of many African nations than the military, and few institutions in Africa are more threatened by AIDS.

At Angola's central military hospital here in the capital, AIDS has surpassed malaria as the leading cause of death, and after the long civil war, the situation will almost certainly worsen.

"With the end of the war, we expect there will be an explosion in numbers," Dr. Francisco Ernesto, the commander of the military health service, said in an interview.

But leaders here are not the only ones who have reason to be alarmed. Africa is figuring in American foreign policy more than at any other time since the end of the cold war, both in terms of economic security and military strategy.

The United States is importing more and more oil from West Africa, particularly Angola and Nigeria, to reduce its reliance on the volatile Middle East. On the other side of the continent, the United States is establishing an antiterrorist command center in the tiny nation of Djibouti and stepping up contacts with Ethiopia and Kenya, all in an effort to build alliances in a region where Al Qaeda has been active, especially in Somalia, Sudan and Yemen.

AIDS in the military will undermine such efforts, and that helps explain why the Pentagon is spending several million dollars this year to help Angola and 20 other African countries begin dealing with the crisis. A new Central Intelligence Agency report on AIDS cites Nigeria and Ethiopia, sub-Saharan Africa's two most populous countries, as crucial American security concerns, and the rising toll on their armed forces is part of the reason.

"A key ingredient of regional cooperation is national militaries that are capable and competent and not dying off because of AIDS," Theresa Whelan, director of the Defense Department's office of Africa policy, said in a telephone interview from Washington.

Angola's civil war made travel around the country difficult and dangerous, and that kept H.I.V. from spreading as much as it has in the rest of southern Africa.

But the war ended this year, after nearly three decades of fighting, and millions of Angolans are on the move, making their way back to villages and towns they abandoned long ago. H.I.V., now estimated to infect 5.5 percent of adults in Angola, will not be far behind, experts say.

"People couldn't move," said Dr. Eric Bing, an assistant professor at Charles R. Drew University of Medicine and Science in Los Angeles and the civilian coordinator of the Defense Department project here. "That's about to change."

Soldiers are already among the sick. Some, infected on missions in foreign capitals like Kinshasa and Brazzaville, will carry the virus to their home villages, passing it on to wives and girlfriends. Others risk being infected as the cycle of transmission gathers pace, and prostitutes and truckers also spread the virus as they ply their trades in areas that had long been inaccessible.

Angola has only to look around the continent, to countries like Ethiopia, Nigeria and South Africa; they are all at peace after long periods of upheaval, and like Angola, all aspire to influence and power on the continent. Yet the stability they are trying to entrench and the ambitions they are trying to realize are threatened by AIDS.

In Nigeria, at least 6 percent of adults are H.I.V.-positive, with the spread fueled by many factors, among them the Nigerian military's emerging role as regional peacemaker.

In Ethiopia, at least 10 percent of the adult population has H.I.V., and the number has been climbing, driven in part by the demobilization of tens of thousands of soldiers after the country's long civil war and more recently after the war against Eritrea.

In South Africa, which has more H.I.V.-positive people than any other country, roughly one in four soldiers are infected, the Ministry of Defense says.

Sergeant Leiria, a 31-year-old commando, has been an Angolan soldier since 1990. Along with missions around the country, he has been sent to Congo and the Congo Republic, countries where Angola has been instrumental in shoring up embattled governments.

These days, he is fighting for his own survival.

He seems fit enough, but he is not. He suffers from thrush, a fungal infection that makes swallowing difficult. He has fought off tuberculosis. He endures debilitating headaches and diarrhea. He is dogged by fever that gives way to chills.

"Usually, from 4 o'clock, I feel too cold," he says. "Even at the moment, I am too cold." Asked how he contracted H.I.V., he says he does not know. It might have been from shaving with a fellow soldier's razor or perhaps from a battlefield blood transfusion, he says.

What about sex? he is asked.

Sent away from home for long stretches, soldiers strike up relationships with local women or prostitutes, though like Sergeant Leiria they often have wives at home. Such liaisons have long spread sexually transmitted diseases like syphilis, and now H.I.V.

"I think it's possible," Sergeant Leiria said. "I cannot lie to you. I walked a lot of places."

Indeed, many soldiers do. The sex is cheap, and in war, life itself seems cheap. Fatalism creeps into their thinking. "People used to say we should enjoy life," Sergeant Leiria said. "If we get it, we get it."

Slowly, though, he and others in the military say, reckless behavior is waning, but too late for Sergeant Leiria.

Sitting in a quiet corner of a downtown hotel, he was dressed in gray jeans, a dark T-shirt and calf-high combat boots that were the only obvious hint of his work. Soon, he said, he will be shedding the uniform altogether. "I want to be demobilized, because of the AIDS, because I need to rest," he said. "I fought a lot and I didn't manage to do almost anything with my life. Now I want to do something in my life. I have children. I have a wife."

It is a painful decision for a man who has known little besides a soldier's life. When he tested positive in 2000, horrified relatives turned their back on him, and he contemplated killing himself.

Since then, he and some of his relatives have come to accept his fate, helped by an AIDS support organization. None of that has helped to arrest the march of the disease. Without consistent access to the anti-retroviral drugs that have made the disease manageable for many people in the West, Sergeant Leiria has little hope. "If I was like other people who have a lot of money — they go abroad to buy medicines, they go to South Africa for treatment — then I could continue," he said.

While Angola is a long way from wielding the broad influence of South Africa or Nigeria, it has fashioned itself into a regional power, and its military, as the fulcrum of power in the country, will remain for now an important engine of the country's aspirations — and a crucial component in the fight against AIDS.

Like other countries, Angola does not know exactly how many soldiers are H.I.V.-positive, and so it is planning a survey with the help of Dr. Bing and other specialists.

The United States military does not enlist anyone who has tested positive for H.I.V., but does not discharge people solely on the basis of infection with the virus unless the symptoms of AIDS render them physically unfit to serve.

Experts say the survey is likely to show that the prevalence is higher than the current estimate of 5.5 percent of adults, and likely to increase. The question is whether Angola will contain the surge. Like many African countries, Angola is too poor to think about providing treatment for most of its H.I.V.-positive people, so for now the efforts focus on prevention.

On the main military base here in the capital, 18 young soldiers were training to teach their colleagues to defend themselves against H.I.V. This is the military's new war, symbolized by the soldier depicted in the class manual, his old weapon — a gun — in one hand, his new weapon — a condom — in the other.

Maj. Fernando Paxião Damião, the doctor in charge of the training, wants to think that his efforts will do some good, but on the future of the armed forces sees little to be cheerful about. "We're going to have an army of sick people," he said.