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.
However, the two strategic rivals are about to become neighbours in a sun-scorched patch of East African desert.
China is constructing its first overseas military base in Djibouti, just a few kilometres 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, US strategists worry that a naval port so close to Camp Lemonnier could provide a front-row seat to the staging ground for US counterterrorism operations in the Arabian Peninsula and North Africa.
"It's like having a rival football team using an adjacent practice field," said Mr 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 terrorist 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 US Navy and abuts Djibouti's international airport, is the only permanent US military installation in Africa.
Beyond surveillance concerns, US 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 Islamist extremism.
Just as important, experts say, the base's construction is a milestone marking Beijing's expanding global ambitions - with potential implications for the US' longstanding military dominance.
"It's a huge strategic development," said Professor Peter Dutton, specialising in 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," Prof Dutton said.
"This is what expansionary powers do. China has learnt lessons from Britain of 200 years ago."
A low-rise encampment built adjacent to a new Chinese-owned commercial port, the 36ha 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.
Chinese officials play down the significance of the base, saying it will largely support anti-piracy operations that have helped quell the threat to international shipping once posed by marauding Somalis.
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.
For US 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 Dr Phillip Saunders, director of the Centre for the Study of Chinese Military Affairs at the National Defense University, referring to the Chinese operation.
"It wouldn't last very long in a war."