With its move to acquire heavy weaponry and top-secret technology, Australia has thrown in its lot with the United States for generations to come — a “forever partnership,” in Mr. Morrison’s words. The agreement will open the way to deeper military ties and higher expectations that Australia would join any military conflict with Beijing.

It’s a big strategic bet that America will prevail in its great-power competition with China and continue to be a dominant and stabilizing force in the Pacific even as the costs increase.

“It really is a watershed moment — a defining moment for Australia and the way it thinks about its future in the Indo-Pacific region,” said Richard Maude, a former Australian security official who is now a senior fellow at the Asia Society Policy Institute.

“It does represent really quite sharp concerns now in the Morrison government about a deteriorating security environment in the region, about China’s military buildup and about China’s willingness to use coercive power to pursue national interests,” he said.