TAIPEI (Reuters) - For China, whose President Xi Jinping is already taking an increasingly muscular approach to claims in the East and South China Sea, the question of Taiwan trumps any other of its territorial assertions in terms of sensitivity and importance.

After eight years of calm in what had been one of Asia’s powder kegs, the landslide election of an independence-leaning opposition leader, President-elect Tsai Ing-wen, has thrust Taiwan back into the spotlight as one of the region’s most sensitive security issues.

Defeated Nationalist forces fled to Taiwan at the end of the Chinese civil war in 1949. China claims Taiwan as its sacred territory, is estimated by Taiwan to aim hundreds of missiles at the island over a narrow stretch of water and has never renounced the use of force to bring it under its control.

China carried out rare live-fire drills in the sensitive strait that separates the two sides in September, though Taiwan’s defense ministry described them at the time as routine.

“She (Tsai) is going to deal with a very tough-minded leader in Beijing,” said Chu Yun-han, a professor at the National Taiwan University.

But Tsai will also have to be accountable to her own constituency, especially the more radical, pro-independence younger generation, Chu added. “That doesn’t give her too much room for maneuver.”

The election in 2008 of the China-friendly Ma Ying-jeou, and then re-election four years later, ushered in an unprecedented period of calm with China, with landmark trade and tourism deals signed.

Tsai’s Democratic Progressive Party (DPP) is at pains to stress its election will not cause a return to tensions. She addressed the issue of China almost immediately upon claiming victory, saying she would strive to maintain the peace, but added she would defend Taiwan’s interests and its sovereignty.

‘BITTER FRUIT’

A supporter of Democratic Progressive Party (DPP) Chairperson and presidential candidate Tsai Ing-wen celebrates to preliminary results at their party headquarters in Taipei, Taiwan January 16, 2016. REUTERS/Damir Sagolj

While China has been relatively measured in its response, repeating its standard line about opposing independence, great uncertainty lies ahead. China’s official Xinhua news agency warned any moves toward independence were like a “poison” that would cause Taiwan to perish.

In an online commentary on Sunday, Wang Hongguang, a lieutenant general and former deputy commander of China’s Nanjing military region, said the People’s Liberation Army was now better prepared than ever for operations against Taiwan.

“The front line forces are like a tiger who has grown wings,” he wrote. “Tsai Ing-wen and her Taiwan independence forces shouldn’t think they’ll get away with it. The mainland will not swallow the bitter fruit of Taiwan independence.”

The outside world should not underestimate the continued importance of Taiwan to the Chinese leadership, said a senior Western diplomat, citing recent conversations with Chinese policymakers on Taiwan.

“Nothing is more important than Taiwan to Beijing.”

Beijing will have to bear in mind the opinion of a Chinese public that has always been brought up never to question Taiwan’s status as an inherent part of China.

On Weibo, China’s answer to Twitter, the popularity of the phrase “use force to unify Taiwan” soared.

“We are just waiting for you to say the phrase ‘Taiwanese independence’,” said one Weibo user.

In the United States, which has no formal ties with Taiwan but is its most important diplomatic and military supporter, Republican presidential candidate Ted Cruz said the election was “a beacon of light to their neighbors yearning to be free”.

“Now more than ever, we must stand with Taiwan and reaffirm our commitment to their security,” he said in a statement.

Taiwan is a key fault line in the Beijing-Washington relationship.

A month before the election, the Obama administration formally notified Congress of a $1.83 billion arms sale package for Taiwan, prompting anger in Beijing which said it would put sanctions on U.S. firms involved.

MISSILE TESTS

A Beijing-based Chinese source, with ties to the People’s Liberation Army and who meets regularly with senior officers, told Reuters the election would have “far-reaching” consequences for China’s ties with Taiwan, and Sino-U.S. relations.

“I’m very worried about what is going to happen now,” the source said, speaking on condition of anonymity. “Things have become much darker.”

Tsai’s election is also an embarrassment to Xi, who held a historic meeting last year in Singapore with Ma, and used the occasion to call for both sides not to let proponents of Taiwan’s independence split them.

China and Taiwan have nearly gone to war three times since 1949, most recently ahead of the 1996 presidential election. Then, China carried out missile tests in waters close to the island hoping to prevent people voting for Lee Teng-hui, who China suspected of harboring pro-independence views. Lee won by a landslide.

Ties were also badly strained when the DPP’s Chen Shui-bian was Taiwan president from 2000-2008 because of his independence rhetoric, even as he tried to maintain positive relations with Beijing.

But then, the DPP did not have a majority in parliament, which constrained its agenda. This time, the DPP has also won a parliamentary majority, which gives it much more leeway to push legislative priorities.

In any case, China does not need to rattle its sabers to pressure Taiwan - Beijing already holds all the economic cards as the island’s most important trade partner and investment destination.

“Taiwan can’t survive without international support,” said Michael Kau, a former Taiwan foreign ministry official and now a senior fellow at Taiwan Foundation for Democracy. “Because our adversary is giant China.”