The Chinese navy is close to building up one of the world’s most powerful submarine fleet, according to the 2019 edition of the U.S. Defense Department’s annual report on Chinese military developments.

The U.S. Navy is struggling to stay ahead.

The People’s Liberation Army Navy is “the region’s largest navy, with more than 300 surface combatants, submarines, amphibious ships, patrol craft and specialized types,” the report explains. “It is also an increasingly modern and flexible force.”

Beijing deploys its submarines in order to “achieve maritime superiority within the first island chain” that runs from Japan to The Philippines “as well as to deter and counter any potential third party intervention in a Taiwan conflict.”

To that end, China by mid-2019 has acquired six nuclear-powered attack submarine, or SSNs, and 50 conventional attack submarines, or SSs. “The speed of growth of the submarine force has slowed and will likely grow to between 65 and 70 submarines by 2020,” according to the report.

The Chinese undersea fleet includes 12 Russian-built Kilo-class SS units, eight capable of launching anti-ship cruise missiles, plus 13 Song-class/Type 039 SS units and 17 Yuan-class/Type 039A diesel-electric air-independent power attack submarines. The Pentagon expects three more Yuans to join the fleet by 2020.

Beijing’s SSN feet includes two Shang I-class/Type 093 SSNs and four Shang II-class/Type 093A SSNs. “By the mid-2020s, China will likely build the Type 093B guided-missile nuclear attack submarine,” the Pentagon report notes. “This new Shang-class variant will enhance the PLAN’s anti-surface warfare capability and could provide a more clandestine land-attack option.

The Chinese undersea build-up is part of a wider, regional submarine expansion. "Potential adversary submarine activity has tripled from 2008 levels, which requires at least a corresponding increase on the part of the United States to maintain superiority," U.S. Navy admiral Philip Davidson said in a March 2019 Congressional testimony.

"There are 400 foreign submarines in the world, of which roughly 75 percent reside in the Indo-Pacific region," Davidson testified. "One-hundred and sixty of these submarines belong to China, Russia and North Korea. While these three countries increase their capacity, the United States retires attack submarines faster than they are replaced."

In December 2016, the U.S. Navy announced it needed 66 attack submarines in order to meet regional commanders' needs. But in early 2019 the fleet had just 51 attack boats. And that number is set to fall.

Owing to a glut of sub production during the 1980s and a years-long gap in submarine production in the 1990s, the Navy possesses large numbers of old submarines, very few middle-age boats and lots of newer ones. A new Virginia-class SSN costs more than $2 billion to build.

Once a sub's nuclear reactor core wears out, usually after around 30 years of operation, the Navy must either decommission the boat or undertake an expensive refueling. Dozens of three-decade-old Los Angeles-class attack subs are likely to decommission in the next few years, shrinking the overall SSN fleet to just 42 boats in 2028.

"Numerically, SSNs remain the furthest from the inventory objective," the Navy stated in its shipbuilding plan for 2020.

Not only would China likely be able to deploy more submarines to the western Pacific than the United States, but the Chinese boats also might be better-suited than the American vessels are to operations in the region's shallow, crowded littoral zones.

The shallow Taiwan Strait, in particular, is inhospitable to big, deep-diving American subs. "While SSNs have enormous advantages over [diesel-powered] SSKs, shallow terrain partially limits the SSNs’ primary advantage: diving deep at high speeds after firing and thereby evade detection," Henry Holst explained in an essay for the U.S. Naval Institute.

In order partially to compensate for its growing disadvantage in submarines compared to the Chinese fleet, the U.S. Navy is experimenting with new, more efficient ways of supporting its subs, and also is buying robotic submarines that could reinforce manned boats on certain missions.

David Axe serves as Defense Editor of the National Interest. He is the author of the graphic novels  War Fix, War Is Boring and Machete Squad.