DJIBOUTI — The two countries keep dozens of intercontinental nuclear missiles pointed at each other’s cities. Their frigates and fighter jets occasionally face off in the contested waters of the South China Sea.

With no shared border, China and the United States mostly circle each other from afar, relying on satellites and cybersnooping to peek inside the workings of each other’s war machines.

But the two strategic rivals are about to become neighbors in this sun-scorched patch of East African desert. China is constructing its first overseas military base here — just a few miles from Camp Lemonnier, one of the Pentagon’s largest and most important foreign installations.

With increasing tensions over China’s island-building efforts in the South China Sea, American strategists worry that a naval port so close to Camp Lemonnier could provide a front-row seat to the staging ground for American counterterror operations in the Arabian Peninsula and North Africa

“It’s like having a rival football team using an adjacent practice field,” said Gabriel Collins, an expert on the Chinese military and a founder of the analysis portal China SignPost. “They can scope out some of your plays. On the other hand, the scouting opportunity goes both ways.”

Established after the terror attacks of Sept. 11, 2001, Camp Lemonnier is home to 4,000 personnel. Some are involved in highly secretive missions, including targeted drone killings in the Middle East and the Horn of Africa, and the raid last month in Yemen that left a member of the Navy SEALs dead. The base, which is run by the Navy and abuts Djibouti’s international airport, is the only permanent American military installation in Africa.

Beyond surveillance concerns, United States officials, citing the billions of dollars in Chinese loans to Djibouti’s heavily indebted government, wonder about the long-term durability of an alliance that has served Washington well in its global fight against Islamic extremism.

Just as important, experts say, the base’s construction is a milestone marking Beijing’s expanding global ambitions — with potential implications for America’s longstanding military dominance.

“It’s a huge strategic development,” said Peter Dutton, professor of strategic studies at the Naval War College in Rhode Island, who has studied satellite imagery of the construction.

“It’s naval power expansion for protecting commerce and China’s regional interests in the Horn of Africa,” Professor Dutton said. “This is what expansionary powers do. China has learned lessons from Britain of 200 years ago.”

Chinese officials play down the significance of the base, saying it will largely support antipiracy operations that have helped quell the threat to international shipping once posed by marauding Somalis.

“The support facility will be mainly used to provide rest and rehabilitation for the Chinese troops taking part in escort missions in the Gulf of Aden and waters off Somalia, U.N. peacekeeping and humanitarian rescue,” the Defense Ministry in Beijing said in a written reply to questions.

In addition to having 2,400 peacekeepers in Africa, China has used its vessels to escort more than 6,000 boats from many countries through the Gulf of Aden, the ministry said. China’s military has also evacuated its citizens caught in the world’s trouble spots. In 2011, the military plucked 35,000 from Libya, and 600 from Yemen in 2015.

As China’s navy has assumed these new roles far from home, its commanders have struggled to maintain vessels and resupply them with food and fuel.

Capt. Liu Jianzhong, a former political commissar of a Chinese destroyer plying the Gulf of Aden, said the lack of a dedicated port in the region took a toll on personnel forced to spend long stretches at sea.

“For six months, we didn’t reach the shore, and a lot of sailors had physical and psychological problems,” he told the state-run China Military Online. To that end, the new base will include a gym, the ministry said.

Professor Dutton said Beijing would most likely try to “acclimatize” the world by using the facility for commercial purposes when it begins operating this year and then gradually increase the number and variety of warships that dock there.

“It will be relatively incremental in the forward deployment of naval power. You are not going to see a Yokosuka,” he said, referring to the base for the United States Seventh Fleet in Japan.

In its written answers, the ministry said that China was not budging from its “defensive” military policy and that the base did not indicate an “arms race or military expansion.”

In recent years, China has moved aggressively to increase its power projection capabilities through the rapid modernization of its navy. Military spending has soared, with Beijing’s defense budget expected to reach $233 billion by 2020, more than all Western European countries combined, and double the figure from 2010, according to Jane’s Defense Weekly. In 2016, the United States spent more than $622 billion on the military, Jane’s said.

These days, Chinese naval vessels, including nuclear submarines, roam much of the globe, from contested waters of the Yellow Sea to Sri Lanka and San Diego.

China’s decision to establish an overseas military installation comes as little surprise to those who have watched Beijing steadily jettison a decades-old principle of noninterference in the affairs of other countries.

The shift is an outgrowth of China’s evolution from an impoverished slumbering introvert to deep-pocketed mercantilist with economic interests across the globe.

Half of China’s oil imports sail through the Mandeb Strait, the choke point off Djibouti that connects the Mediterranean Sea and the Indian Ocean. Across Africa, state-owned companies are investing tens of billions of dollars in railways, factories and mines.

And the millions of Chinese citizens who live and work overseas have come to expect that the government will look out for their interests — a point driven home in recent years when Beijing was forced to rescue Chinese nationals from strife-torn Libya and Yemen.

“The facility in Djibouti is a very interesting lens through which to view China’s growing capabilities and ambitions,” said Andrew S. Erickson, an expert at China’s maritime transformation at the Naval War College and the editor of the book “Chinese Naval Shipbuilding.”

“Not only will it give them a huge shot in the arm in terms of naval logistics, but it will also strengthen China’s image at home and abroad.”

A low-rise encampment built adjacent to a new Chinese-owned commercial port, the 90-acre base is designed to house up to several thousand troops and will include storage structures for weapons, repair facilities for ships and helicopters, and five berths for commercial ships and one for military vessels.

At the base’s front gate recently, Chinese workers in construction helmets waved away a reporter who tried to ask questions. China’s Defense Ministry declined a request to tour the site.

American officials say they were blindsided by Djibouti’s decision, announced last year, to give China a 10-year lease for the land. Just two years earlier, Susan Rice, the national security adviser under President Barack Obama, had flown here to head off a similar arrangement with Russia.

Shortly afterward, the White House announced a 20-year lease renewal that doubled its annual payments for Camp Lemonnier, to $63 million, and a plan to invest more than $1 billion to upgrade the installation.

If the Pentagon’s current base restrictions are any guide, American and Chinese troops are unlikely to be sharing beers any time soon. American officials, citing possible security threats, keep most personnel confined to the 570-acre rectangle of scrubland, which is a 10-minute drive from the center of Djibouti city. It is a policy that stirs some discontent among those who often spend yearlong stints at Camp Lemonnier without venturing outside.

By contrast, French military personnel can often be seen jogging through the city and socializing with locals. Americans who work for the United States Embassy also live in the community and say they feel little threat to their safety.

Life on base can be monotonous, broken up by visits to the fitness center or meals at the camp’s Subway sandwich outlet. Capt. James Black, the camp’s commanding officer, said one of his primary challenges was to provide salubrious distractions for those stationed here. The distractions include free Wi-Fi, a movie theater, Texas Hold ’em tournaments and the occasional soccer match with Italian and German troops.

“We’re like a landlocked aircraft carrier,” Captain Black said during a recent tour of the installation, which is blasted in summer by broiling heat. “Part of my job is to create opportunities to give people a break and attend to their mental health needs.”

Local residents also crave more face time with the Americans. Some say Camp Lemonnier personnel could play a more active role in helping to alleviate Djibouti’s crushing poverty by building schools, painting hospitals or simply taking part in language exchanges.

Others, like Mohamed Ali Basha, the owner of a Yemeni-style restaurant that serves grilled fish and massive discs of baked flatbread, said he would welcome business from military personnel.

“I don’t understand why the Americans are so obsessed with security here, but I would be happy to close the restaurant for them if they would come,” Mr. Basha, 26, said. “Just call in advance.”

In interviews, Djiboutian officials expressed little concern that two strategic adversaries would be sharing space in a country the size of New Jersey. It helps that the Chinese are paying $20 million a year in rent on top of the billions they are spending to finance critical infrastructure, including ports and airports, a new rail line and a pipeline that will bring desperately needed drinking water from neighboring Ethiopia.

Critics say the surge of loans, which amount to 60 percent of the country’s gross domestic product, raises concerns about China’s leverage over the Djibouti government should it fall behind on debt payments.

“Such generous credit is itself a form of control,” said Mohamed Daoud Chehem, a prominent government critic. “We don’t know what China’s intentions really are.”

But on the city’s dusty, potholed streets, most people are pleased to see China joining the club of a half-dozen foreign militaries that have a presence here, among them Japan, Italy and Britain. Also here is a large contingent of French soldiers who stayed on after 1977, when the colony formerly known as French Somaliland gained independence.

Abdirahman M. Ahmed, who runs Green Djibouti International, an environmental social enterprise, said many people viewed foreign militaries as a stabilizing force, given their country’s diminutive size, its lack of resources and the potential threats from neighbors like Ethiopia, Somalia and Eritrea, where expansionist sentiments continue to burble.

“We don’t see any problem having the Chinese here,” he said. “They provide revenue and help play a deterrence to those who would love to annex Djibouti.”

The plethora of foreign troops, some say, also served as a bulwark against the jihadist violence that has destabilized other countries in the region. Djibouti, whose population of 900,000 embraces a moderate form of Sunni Islam, has not been entirely spared: In 2014, a double suicide bombing at a downtown restaurant popular with foreigners killed a Turkish national and wounded 11 people. The Shabab, the Somali-based militant group, later claimed responsibility, saying the attack was motivated by the presence of so many Western troops in Djibouti.

For American military strategists, the security implications of the Chinese base are unclear, though practically speaking, many experts say the military threat is minimal.

“A port like this isn’t very defensible against attack,” said Philip C. Saunders, director of the Center for the Study of Chinese Military Affairs at the National Defense University. “It wouldn’t last very long in a war.”