China has never renounced the use of force to take back what it deems a wayward province, and Taiwan's defence ministry says China has more than 1,000 missiles directed at the island.
"Strengthening underwater combat capabilities is most needed in Taiwan's defense," Tsai said during a tour of a submarine at the southern naval port of Zuoying, about 350 kilometers (218 miles) from the capital, Taipei.
"This is a problem everyone recognizes," she added. "We have been unable to solve this in the past. As commander of the armed forces, I am determined to solve this problem."
But the rare appearance of two of Taiwan's four submarines at the event also spotlighted the island's slow, sometimes stalled efforts, to upgrade key defence equipment.
The black-hulled vessel half-submerged in the water that Tsai visited has been in service for nearly half a century.
"Making a submarine isn't the problem," said Gao Chung-hsing, vice president of the National Chung-shan Institute of Technology, a quasi-defence ministry agency reponsible for military research and development.
"It is making what kind of submarine that is the problem."
To build an advanced submarine, for instance, Taiwan, which has never before built such a craft, will have to rely on foreign technology to resolve issues such as integrating the hardware with various electronic systems, defence experts say.
Such foreign support is critical to Taiwan's effort, which was allocated a four-year budget of T$3 billion ($99 million) for its design contract phase from 2016, Taiwan defence officials and experts say.
Two submarines in Taiwan's fleet date from the era of World War Two, were bought from the United States, and are used mainly for training, while the other two, bought from the Netherlands in the 1980s, first saw service in the 1970s.
Although the United States agreed to sell Taiwan eight diesel electric submarines in 2001, the purchase never went through, beset by hurdles ranging from budget issues and lack of consensus in Taiwan to changing U.S. policy priorities.
Washington has begun considering a big, new arms package for Taiwan, a move sure to anger China.
This week, officials in Taiwan fretted that a planned summit meeting between U.S. President Donald Trump and Chinese President Xi Jinping could sacrifice Taiwan's interests.
Tsai, who leads the independence-leaning ruling Democratic Progressive Party (DPP), has never conceded to Beijing's view that Taiwan is a part of China, although she has soft-pedalled the issue since taking office in May last year.
In December, Taiwan briefly celebrated a diplomatic coup when Trump, then president-elect, took a congratulatory phone call from Tsai and raised questions about whether he would stick with the four-decade-old "one China" policy.
Trump changed tack last month, however, and agreed to honour the "one China" policy during a phone call with Xi, reviving the island's concerns about its vulnerability.
"If there was no threat across the Taiwan Strait, then we do not have to purchase arms," Defence Minister Feng Shih-kuan told Taiwan's parliament on Monday.
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