The first phase of the Chinese invasion of Taiwan could begin with a naval and air blockade along with a series of cyberattacks, missile strikes, and electronic jamming, followed by round after round of bomber strikes. The second phase could feature amphibious landing operations on some of Taiwan’s smaller islands, a few of which sit just miles from the Chinese mainland. The third and final phase could involve the immensely difficult task of capturing the main island of Taiwan—no easy task given its tight military relationship with America, its professionally trained soldiers, and its rugged mountains and dense jungles.

According to leaked and restricted Chinese military documents gathered and analyzed by scholar Ian Easton for his forthcoming book The Chinese Invasion Threat, published Tuesday, many in Beijing recognize the difficulty of seizing Taiwan. And yet, capturing Taiwan—or ‘reunification,’ in the ruling Chinese Communist Party’s parlance—is one of the main reasons for the extensive military buildup of the Party’s People Liberation Army over the last few decades. “Only by military occupying The Island,” Easton cites a restricted-access PLA field manual as saying, can we “totally end the long military standoff across the Strait.”

One important and little-known element of China’s war plans that Easton stresses is psychological warfare, or political attacks. PLA documents, Easton says, “portray the battle of Taiwan as the final act in an unresolved civil war and place great emphasis on winning (or at least weakening) hearts and minds.” Easton, a research fellow at the D.C. think tank Project 2049, which receives some of its funding from the Taiwanese government, writes, “while it may seem unbelievable to most foreigners, officers in the Chinese military are constantly studying and practicing plans for the invasion of Taiwan.”

Sure, you might say, part of the job of a professional military is to prepare contingency plans for a whole host of conceivable scenarios—which in the PLA’s case probably includes situations like a U.S. invasion of China, the collapse of North Korea, a war prompted by territorial disputes in the South and East China Seas, and another border conflict with India, among others. But Taiwan is different. Somewhat similar to the United States’ desire for territorial integrity after the Civil War, or its western expansion earlier in the 19th century, the reunification of Taiwan looms large in China’s vision of its successful future.

If Taiwan dominates China’s regional military plans, you wouldn’t know it from the conversation in Washington. For the last nine months, North Korea has dominated American news coverage about Asia, as Donald Trump and Kim Jong-un have escalated tensions by hurling verbal salvos across the Pacific. On Sept. 21, Kim called Trump a “mentally deranged U.S. dotard.” Trump, who earlier that week in a UN speech threatened to “totally destroy” North Korea, on Sept. 24 tweeted a warning that “they won’t be around much longer.” And it’s not just words: On Sept. 3, North Korea conducted its sixth nuclear weapons test. The U.S. responded by stepping up U.N. sanctions, and in a show of force, flew U.S. bombers and fighter jets near North Korea’s east coast on Sept. 23. The public alarm over the potential for war with North Korea is warranted. Depending on how it unfolded, such a war could lead to the death of thousands or even millions of people in North Korea, South Korea, China, and the United States.

But war between China and Taiwan could be equally devastating. There are three reasons to believe this scenario, in the next ten years, is at least as likely as war between the United States and North Korea. For one, the goal of “liberating” Taiwan is the paramount foreign policy concern of Beijing. And it has been a top concern since the end of the 1945–1949 civil war between Mao Zedong’s Communists and Chiang Kai-Shek’s Nationalists, when Chiang and his people fled to the island, setting up what the West viewed as China’s legitimate government until the 1970s. (Because Beijing insists Taiwan is part of China, it does not call Taiwan an international issue.)

Taiwanese reunification and independence is such a sensitive topic on the mainland that any polling on the issue is suspect. Anecdotally, however, in the dozens of conversations I’ve had with Chinese citizens about Taiwan over the last 15 years, many of them supported reunification—some with force, if necessary. The Communist Party ties some of its legitimacy to its ability to follow through on its long-standing promise to re-absorb Taiwan—it risks a loss of legitimacy if it continues to fail. A healthy democracy of 24 million people, Taiwan belies the party’s implicit argument that Chinese people need an authoritarian government in order to flourish.

Secondly, the benefits to China of successfully absorbing Taiwan far supersede the benefits of the United States of neutralizing North Korea. It’s very unlikely that North Korea would ever strike the United States: Its leaders seem rational enough to realize that an attack on U.S. soil, however small, would be an act of regime suicide. If the United States successfully replaced Kim with a regime more supportive of U.S. interests, or even more advantageously, facilitated the reunification of the Korean peninsula under a Western-friendly government in Seoul, that would improve the United States’ ability to project power in Asia and constrain the rise of China. Still, North Korea is a distraction, not an existential issue, for China.

Beijing’s successful occupation of Taiwan, on the other hand, would greatly improve its prospects for regional domination, and undermine the United States’ position in Asia by removing America’s democratic ally Taiwan and weakening Japan. And it would ensure Beijing’s ability to maintain its trade links in the Western Pacific in the face of a U.S.-organized blockade.

Easton cites a line from the restricted Chinese document on Japanese air defenses: “As soon as Taiwan is reunified with Mainland China, Japan’s maritime lines of communication will fall completely within the striking range of China’s fighters and bombers.” Because of the mutual enmity between Japan and China—and the persistent desire for revenge from the Chinese public for the atrocities Japan committed in China during the 1930s and 1940s—another war between China and Japan over the next few decades is not as far-fetched as it may sound. And if Taiwan were occupied, its strategic location in the East China Sea would greatly aid Beijing’s ability to harry southern Japan. (During World War II, U.S General Douglas MacArthur called the island an “unsinkable aircraft carrier.”)

What could set off a Chinese invasion? Beijing has long threatened to invade if Taiwan declares independence—though what exactly that means, like many issues involving Taiwan’s status, is murky and open to interpretation. The ruling Democratic Progressive Party (DPP), one of Taiwan’s two main parties, skirts this by claiming in a 1999 resolution there is no need to declare independence because “Taiwan is a sovereign and independent country.” Taiwan’s President Tsai Ing-wen reiterated this phrase in October 2016, prompting China’s Taiwan Affairs Office to retort, “all secessionist attempts to seek ‘Taiwanese independence’ are doomed to failure.” On Sept. 26, Taiwan’s newly appointed Premier William Lai said he “advocates Taiwan independence,” and repeated Taipei’s stance that the Republic of China, as Taiwan is officially known, was already independent, and thus there is no need for him to declare it. If it so chooses, Beijing could start considering statements like Tsai’s and Lai’s as de facto declarations of independence—and respond with military force. Chinese military experts I spoke with said that if war does come, it will likely be after Beijing has at least several more years of improving its military capabilities—possibly sometime around 2021, the 100th anniversary of the founding of the Chinese Communist Party. (These experts stress it’s impossible to predict if and when Beijing would actually go to war with Taiwan.)

Or Beijing could wait for a moment of relative calm. PLA doctrine, Easton writes, likely favors “a minimal warning, rapid invasion campaign that employs deception and surprise to land on the island and overrun Taipei, securing the government’s capitulation before U.S.-led coalition forces could decisively engage.”

While the United States, along with nearly all Western powers, doesn’t have formal diplomatic relations with Taiwan, the United States has long had a close relationship with the island. The United States continues to sell Taiwan weapons, including a planned $1.42 billion arms sale announced in June. And it maintains a posture of strategic ambiguity about whether it would defend Taiwan in the face of a Chinese attack.

It’s difficult to know if Trump—who took an unprecedented phone call from Tsai after his election victory—would come to Taiwan’s aid in the event of a military conflict. A former high-ranking Obama administration official once told me “it’s useful” for Beijing to believe the United States would defend Taiwan from a Chinese attack—but he added that it’s “profoundly important” for the United States to manage the relationship so it doesn’t get to that point. His words hold true today.

The Taiwan scenario is so potentially devastating not only because of the potential of sparking a war between the United States and China. Taiwan is a more formidable foe than North Korea, and its geography—an island nation with dense jungles and mountains covering parts of the country—would make it difficult to invade and hold.

Throughout his book, Easton admits two important caveats. “All of the PLA’s internal materials on the invasion of Taiwan are theoretical, and its campaign plan is based on imagined conditions and assumptions,” he writes. He cites Prussian Field Marshal Helmuth von Moltke’s famous line “no plan survives contact with the enemy.”

The other, most important one, is that although Easton accessed restricted material he couldn’t find the Joint Island Attack Campaign, a document, or series of documents, that “appears to be highly centralized and updated regularly based on the latest intelligence, weapons production, and lessons learned from exercises and training.” The most crucial information about an invasion of Taiwan remains highly classified, and likely inaccessible to all but those at the top of the Party and the PLA—and possibly, depending on how successful foreign intelligence arms have been in China, with the CIA or Taiwan’s National Security Bureau as well. To his two caveats I would add another: The majority of his important PLA source material comes from 2014, early in Party Secretary Xi Jinping’s term. Plans have almost certainly evolved since then. Still, Easton’s book paints a much fuller picture than what is currently available in English about China’s plans for Taiwan. And it’s a reminder of the deadly seriousness with which Beijing views the island. (Easton’s sympathies clearly lie with the Taiwanese. For example, he refers to the date of a Chinese invasion of Taiwan as “Z-Day,” the term Winston Churchill used when discussing Adolf Hitler’s plans for a Nazi invasion of England.)

Plenty of things could derail China’s future plans for a Taiwanese invasion. Public opinion in Taiwan could shift drastically in support of the mainland—or the mainland could evolve into a democratic polity that enables Taiwan to declare independence. China’s economy could crash, and Beijing could decide to shift future resources away from the military, reducing the likelihood of a successful invasion. Or the two sides could maintain the current rickety status quo—punctuated by occasional crises, that don’t lead to war.

And yet, as China’s military continues to strengthen relative to the U.S. and Taiwanese militaries, China’s chance for a successful of invasion grows. On Sept. 27, Chairman of the Joint Chiefs of Staff Gen. Joseph Dunford told Congress, “I think China probably poses the greatest threat to our nation by about 2025.” In response, the CSIS China scholar Zack Cooper summed up U.S policy toward China: always important, never urgent.