SAN DIEGO — President Biden took his most aggressive step yet on Monday to counter China’s military expansion in the Asia-Pacific region, formally unveiling plans with Britain and Australia to develop and deploy nuclear-powered attack submarines.

Standing in front of the U.S.S. Missouri, a nuclear submarine, at the Point Loma naval base in San Diego, Mr. Biden and the leaders of the other two countries described the naval partnership as a critical way to confront China at a time of heightened tension with Beijing. It will create, U.S. officials said, a “nuclear stewardship” among the allies.

“The United States has safeguarded stability in the Indo-Pacific for decades for the enormous benefit of nations throughout the region,” Mr. Biden said, adding, “We’re showing again how democracies can deliver our own security and prosperity, and not just for us but for the entire world.”

For the first time in 65 years, Mr. Biden said, the United States will share the technology at the heart of its nuclear submarines, allowing Australia to build powerful war machines that will grow into fleets capable of facing off with Chinese vessels in the South China Sea and other waters. Initially, Australia will buy three submarines like the Missouri, and will eventually build a new version, called the AUKUS, with British and American help.