The US Navy has its eyes on a new submarine base that will help it keep track of China's growing undersea fleet

• As part of the AUKUS agreement, US and British subs will operate out of western Australia by 2027.
• That location will allow allied subs to spend more time operating in the Pacific and Indian oceans.

US and British submarines will soon be operating out of Australia, giving the allies greater reach and presence in the Indian and Pacific oceans, where undersea activity by friends and foes alike has been intensifying.

In March, President Joe Biden and his British and Australian counterparts announced a timeline for Australia to acquire new nuclear-powered submarines, first buying US-built subs in the early 2030s and then receiving the first Australian-built boats by the early 2040s.

In the next few years, however, US and British subs will visit Australia more frequently, and as soon as 2027, those navies will begin basing attack submarines — one British and as many as four from the US — at HMAS Stirling, an Australian navy base near the city of Perth on the Indian Ocean coast.

Designated Submarine Rotational Forces-West, the subs will rotate through HMAS Stirling rather than being permanently assigned, and officials say their presence will help develop Australia's capacity and capability to operate its own nuclear-powered subs.

"This rotational force will help build Australia's stewardship," a senior Biden administration official said ahead of the March 13 announcement. "It will also bolster deterrence with more US and UK submarines forward in the Indo-Pacific."

The AUKUS deal comes as each country's main rivals — China and Russia — expand their submarine fleets and operations in the Pacific.

Russia continues to deliver new submarines to its Pacific Fleet, including two in one day in October. One of those was the latest of the Yasen-class guided-missile subs, which worry US commanders because their ability to operate more quietly than other Russian subs could challenge the US Navy's ability to track them.

Yasen-class subs operating in the Atlantic and the Pacific present "a dual-flank challenge" for the US, the admiral in charge of the Office of Naval Intelligence said earlier this year. The head of US Northern Command, which is responsible for North America, said this spring it was "just a matter of probably a year or two" until Yasen-class subs were "a persistent threat" to the US mainland.

China's submarines are generally not as advanced as Russia's, but its navy has put "a high priority" on modernizing them, the US Defense Department said in its most recent report on the Chinese military, though that fleet will "grow modestly" as China focuses on improving the force, adding new technologies, and expanding its shipyards.

Nonetheless, China's undersea force has made notable developments. Its newest Yuan-class diesel-electric subs are quiet and have more advanced sonars and "might be actually pretty good at anti-submarine warfare," said Bryan Clark, a senior fellow and director of the Center for Defense Concepts and Technology at the Hudson Institute.

The Defense Department report also says China's six operational Jin-class nuclear-powered ballistic-missile subs are likely already conducting "near-continuous at-sea deterrence patrols," a sign that China's submarine force continues to improve its operational capabilities.

The head of US Strategic Command, which oversees US nuclear forces, told lawmakers this spring that those Jin-class subs are being armed with new third-generation JL-3 ballistic missiles that can reach the US from the South China Sea, a first for Chinese subs.

For the US Navy, those developments make the ability to base subs closer to the Western Pacific a greater priority.

Twenty-five of the US Navy's 49 attack submarines are based in the Pacific, according to Rear Adm. Jeffrey Jablon, commander of the US Pacific Fleet submarine force, and the Navy has moved some of them farther west. In November 2021, Jablon announced that the number of subs based in Guam would increase from two to five; the fifth arrived in March 2022.

Jablon also announced in November 2022 that the Navy would increase investment in its submarine support facilities in Guam in order to expand its operational capability there.

Like Guam, operating from Australia would put more subs closer to the Western Pacific and allow those subs to spend more time at sea. Subs based at Perth could do longer-term deployments, operating in the region for up to a year "as opposed to a normal deployment, which is six months where you spend a month transiting either way from the US West Coast and you basically get four months on station," Clark, a former US Navy submarine officer, told Insider.

Travel time from the West or East Coast "is obviated if you're staging from" Perth, Harry Harris Jr. a retired US admiral and former commander of US Pacific Command, said in response to a question from Insider at a think-tank event on March 30.

Harris Jr. called SRF-West "a positive" and said "it certainly increases our day-to-day presence and numbers in the Indian Ocean [and] certainly in the Western Pacific."

Forward-deployed subs also usually have a higher operational tempo, and with roughly a half-dozen subs in Guam and three to four in Perth, Clark said, "you're going to have on any given day easily a half-dozen to eight submarines underway, driving around, which is more than we have today."

"The end result could be almost doubling the US submarine presence in the Western Pacific," Clark added.

Chinese submarines now appear to operate mostly in the "near seas" off of the Chinese coast, especially its missile subs in the "bastion" of the South China Sea, where China's naval and air forces offer additional protection.

The development of more advanced subs, particularly nuclear-powered subs capable of staying at sea for longer, may allow the Chinese navy to send more subs into more distant waters for longer periods, including into the Indian Ocean, where China's military is likely to "significantly increase" its presence in the coming years as part of a strategy that calls for "an ability to project power and fight in distant seas," according the Office of Naval Intelligence.

Chinese subs heading to the Indian Ocean are likely to use straits in the Indonesian archipelago that are closer to Australia, which are deeper and allow those subs to remain submerged, according to Collin Koh, a research fellow focused on naval affairs at the S. Rajaratnam School of International Studies in Singapore.

"Having UK and US subs based in Perth can provide a vantage point to range operations into those areas," Koh said.

AUKUS' ambitions for increasing the number of submarines in the Indian and Pacific oceans still face hurdles. Australia will have to develop the institutional knowledge and facilities to maintain nuclear-powered subs, which it has never done before, and its future sub orders may further tax US shipyards, which are straining to deliver subs ordered by the US Navy. Delays on sub repairs are also challenging the US Navy's ability to keep subs in the water.

But additional British and eventually Australian submarines in the region will ease the burden of tracking China's increasingly active sub fleet, a task that now falls largely on US submarines. "This may compel Beijing to seek to minimize the risks of detectability and trackability of its submarines," Koh told Insider.

Until the Chinese military improves its ability "to project and sustain a wider array of military capabilities" to help keep those allied subs at bay, its subs may continue to focus on operations within range of friendly naval and land-based forces, Koh said. "This means the AUKUS program may complicate Beijing's efforts to range submarine operations out into the far-seas."