A popular question for journalists to ask America's top military commanders and national security officials is, "What keeps you up at night?"

Increasingly, the answer is the spectre of war with China.

"I have a fear that we will come to military conflict with China within the decade, and it will be when they make their move on Taiwan," former Secretary of State Rex Tillerson said in a January interview with Foreign Policy.

Suppose Taiwan has any doubt about what Chinese President Xi Jinping has in mind for the self-governing island roughly 100 miles off China's east coast. In that case, it need only look at what's happening now in Hong Kong, where Beijing has crushed street protests and used a harsh national security law to eliminate dissent and silence the media and political opposition.

China's "one country, two systems" policy was supposed to guarantee Hong Kong semi-autonomy for 50 years after the end of British rule in 1997. At one time, it was considered a model for how Taiwan might one day reunite with the mainland.

Any such peaceful resolution to Taiwan's status now seems more remote than ever. Xi is losing patience with what China considers a renegade province that needs to come home.

"Xi has committed himself to bringing Hong Kong and Taiwan both back — integrating them back into China, both while he is still in office," said Robert Gates, the former defence secretary and CIA director, last month in an interview with the Washington Post. "This would sort of put him in the same pantheon as Mao, as having finished the revolution of 1949."

China's years long military build up, which includes "carrier killer" cruise missiles, hypersonic weapons, and an expanded nuclear arsenal, all have one apparent goal in mind.

"Xi's strategy now is clear: to vastly increase the level of military power that China can exert in the Taiwan Strait, to the extent that the United States would become unwilling to fight a battle that Washington itself judged it would probably lose," wrote former Australian Prime Minister Kevin Rudd in an essay in Foreign Policy. "Without U.S. backing, Xi believes, Taiwan would either capitulate or fight on its own and lose."

Key to China's strategy is its expansive claims in the South China Sea and its militarization of a string of islands, both natural and human-made, to prevent the kind of Pacific island hopping that helped the U.S. defeat Japan in World War II.

"What China is trying to do is to create in the South China Sea a barrier that would make it just far too costly for us to come to any ally's defence," former national security adviser and retired Lt. Gen. H.R. McMaster told Congress this month.

In testimony before the Senate Armed Services Committee, McMaster called Taiwan "the most significant flashpoint" that could lead to a large-scale war with China and sees the period of "greatest danger" beginning in 2022 after the conclusion of the Beijing Winter Olympics.

"I think that has to do with really Xi Jinping's belief that he has a fleeting window of opportunity that's closing, and he wants to, in his view, 'make China whole again,'" he said.

"I just think that is where it is headed," Tillerson agreed. "I think it's Xi's plan to raise the stakes so significantly to U.S. military losses that the American people will say, 'Wait a minute, we're going to incur thousands of casualties to save Taiwan. Why would we do that?'"

The Pentagon has now begun a furious effort to shore up Taiwan's defences while racing to develop new weapons and tactics of its own to counter China's growing military might, including swarms of manned and robotic ships and aircraft, missile defences, and space-based radars, all designed to give China pause.

"We must convince Beijing that the costs to achieve its objectives by military force are simply too high," said Adm. Philip Davidson, commander of the U.S. Indo-Pacific Command, who this month submitted a congressionally mandated shopping list of military upgrades he said will be urgently needed over the next five years, with a $27.3 billion price tag.

"Make no mistake about it: China seeks a new world order," Davidson said in recent remarks at the American Enterprise Institute. "The period between now and 2026, this decade, is the time horizon in which China is positioned to achieve overmatch in its capability and when Beijing could likely choose to forcibly change the status quo in the region. And I would say the change in that status quo could be permanent."

Unlike Korea, where the U.S. is bound by treaty to defend the South in the event of an attack, the U.S supports Taiwan by supplying defensive weapons capable of defeating an air and sea assault by Chinese forces while pursuing a delicately calibrated policy of "strategic ambiguity." It is designed to keep Beijing guessing whether the U.S. military and its allies would come to Taiwan's defence if China were to launch a cross-strait invasion. Gates, who was the defence secretary to former Presidents George W. Bush and Barack Obama, is among a growing number of national security experts who believe it may be time to replace that nebulous policy with something resembling "strategic clarity."

"Given where President Xi is headed ... we ought to think seriously about whether it's time to abandon our long-time strategy of strategic ambiguity toward Taiwan and basically tell the Chinese that if unprovoked, they take actions against Taiwan, the United States will be there to support Taiwan," Gates said. "I think this is a really dicey situation."

"This will be the decade of living dangerously. No matter what strategies the two sides pursue or what events unfold, the tension between the United States and China will grow, and competition will intensify; it is inevitable. War, however, is not," wrote Rudd, who is now president of the Asia Society in New York.

Retired Adm. James Stavridis, NATO'S former supreme commander in Europe, agreed that there is still time to establish guardrails with China, including by finding ways to cooperate on mutually beneficial issues, such as climate change, while managing the highly competitive relationship.

To help underscore the risk of the U.S. and China sleepwalking into a war that would be enormously costly for both sides, Stavridis and co-author Elliot Ackerman have written 2034: A Novel of the Next World War, inspired by classic Cold War fiction that imagines how miscalculation on both sides results in catastrophic consequences.

"It's cautionary fiction," not "predictive fiction," Stavridis said. "If you stop and think about it, part of the reason we never ended up going to war with the Soviet Union was [that we] had a lot of Cold War literature that tells us how terrible that outcome would be. Think Dr. Strangelove, On the Beach, Fail-Safe, and The Bedford Incident."

The fictional account, which is replete with technically accurate scenarios that a future war with China could pose, offers real-world lessons about the danger of human failures and the tendency to miss de-escalation "off-ramps," along with a vivid portrait of life and death issues that war-planners of today will need to consider tomorrow.

One of the book's lessons: "We need to better protect our cybersecurity," Stavridis said. "And if you didn't believe me two months ago when I said that, I hope you now believe me following the SolarWinds hack."

The book also imagines how India and other East Asian allies could provide a crucial counterbalance to a rising China in a future war.

"Get close to India," Stavridis said. "It's a good balance to China that would help create a better set of outcomes."

The book cites and illuminates two quotes from the ancient sixth-century Chinese general and revered military strategist Sun Tzu: "The supreme art of war is to subdue the enemy without fighting," and later, "When on death ground, fight."

"We want to make sure we don't push China into a corner, and China wants to make sure they don't push us into a corner," Stavridis said. "And that's really the story of 2034."