SEOUL/TAICHUNG (NYTIMES) - On a cloudy day last month, thousands of soldiers massed on a beach in central Taiwan for the culmination of five days of exercises intended to demonstrate how the island's military would repel an invasion from China.

Jets, helicopters and artillery and missile batteries fired live ammunition at targets offshore, sending plumes of sea spray into the air.

Then, a few hours later, a military helicopter taking part in the same exercise crashed at an airfield farther up the coast, killing two pilots and casting a shadow over the show of force.

It was the latest in a string of deadly mishaps, including a crash in January that killed the military's top commander, that has given new urgency to the debate over Taiwan's readiness to defend its 24 million people - with or without the help of the United States.

"I have to be honest: Taiwan's military needs to improve a lot," Mr Wang Ting-yu, a member of the parliament's foreign affairs and defence committee, said in a telephone interview.

Taiwan's leaders have been moving to shake up the military and increase spending.

Military tensions across the Taiwan Strait have surged in recent months as Taiwan has increasingly become a focal point in the confrontation between China and the United States.

Last week, the People's Liberation Army of China held a fresh round of live-fire exercises - an unusually concentrated training schedule that the state news media said was directed at Taiwan and the United States.

The latest involved a test firing of four medium-range ballistic missiles into an area of the South China Sea near Hainan on Wednesday.

The barrage came a day after China accused the Americans of flying a U-2 spy plane over one of the exercises, calling it a "naked provocation".

China's authoritarian leader, Mr Xi Jinping, has long threatened to use force, if needed, to prevent any movement toward formal independence for Taiwan, a self-governing democracy.

China has stepped up those warnings ever since Taiwan's president, Ms Tsai Ing-wen, won re-election in January by vowing to protect the island's sovereignty, defeating a candidate viewed as more conciliatory.

That has raised fears that Mr Xi could feel compelled to act aggressively, as China has from the South China Sea to the border with India.

Chinese aircraft and warships have repeatedly menaced Taiwan's airspace and territorial waters in recent months, while officials have taunted its military, comparing it to "an ant trying to shake a tree."

"The likelihood of a military clash is much higher than before," said Mr Lin Yu-fang, a Taiwanese former legislator from the opposition party that ruled the island for decades, the Kuomintang.

Ms Tsai has responded to China's muscle-flexing by pressing ahead with military changes.

She has moved to revamp Taiwan's military doctrine and strengthen its reserves, a force that would be crucial to defending the island in the event of an invasion.

Her government announced this month that it would increase Taiwan's defence budget by 10 per cent, on top of a 5 per cent increase the year before.

That would raise military spending to more than 2 per cent of gross domestic product - a level that US President Donald Trump has derided many Nato allies for not sustaining, and the highest since the 1990s.

Taiwan also finalised a deal announced last year to buy 66 American F-16 fighter jets, worth $62 billion (S$84 billion) over the next 10 years.

"Grovelling will not bring national security," Ms Tsai, wearing combat fatigues and body armour, declared at the beach exercise in July.

She argued that only a strong defence, not conciliatory gestures to China's Communist leadership, would preserve Taiwan's de facto independence.

For Ms Tsai, the island's defence has become an especially fraught issue.

She faces criticism for not doing enough to support Taiwan's military, while she risks antagonising Beijing for doing too much.

A crucial consideration is the role that the US might play in a conflict.

By law, the US is committed to providing Taiwan with the support necessary to defend itself, a point reiterated by Secretary of Defence Mark Esper in a recent talk.

Yet it is far from clear whether the US would risk a broader confrontation with a nuclear-armed China, meaning Taiwan cannot count on it as a matter of strategy.

Ms Tsai's predecessor, Mr Ma Ying-jeou, has accused her of clinging to hopes that as long as Taiwan put up an initial defence, the US would intervene on the island's behalf, a scenario he considered impossible.

He did not elaborate, but Ms Tsai said in an interview last year that Taiwan would be able to hold out for 24 hours and then China would face international pressure.

"I feel as a president, one should not tell our people how many days we can last," said Mr Ma, who pursued a policy of détente with China during his two terms from 2008 to 2016 and has urged Ms Tsai to do the same.

"We need to tell our people we can stop the war from happening."

For decades, Taiwan's security was assured by the island's military capabilities, but China's efforts to modernise its forces have upended the balance of power.

China now has "an array of options for a Taiwan campaign, ranging from an air and maritime blockade to a full-scale amphibious invasion", according to a 2019 Pentagon report on the Chinese military.

The report acknowledged the challenges that the Chinese army would face in such an attack but said China's buildup "has eroded or negated" many of Taiwan's advantages.

Those include the island's geography and the technical superiority it once had from buying American and other foreign weaponry.

Taiwan's summer exercises, like the large purchases of American hardware, are intended to demonstrate the military's ability to counter Chinese aggression, even if outnumbered and outgunned. The authorities are investigating the cause of last month's helicopter crash.

The F-16 jets are meant to replace Taiwan's ageing air force, which has suffered a number of accidents in recent years, and that could help challenge Chinese aircraft for domination of the skies.

At the same time, Taiwan cannot afford to compete plane for plane, ship for ship, tank for tank against the far larger People's Liberation Army, according to military analysts.

They argue resources would be better used on capabilities that would slow or even cripple an invading force.

Those include sea mines, submarines and missile systems that could destroy Chinese aircraft and warships before they reached the island.

Others have suggested training units for guerrilla warfare to grind down conventional forces of the type the Chinese would land in an invasion, replicating a strategy used by smaller countries facing larger adversaries, like Estonia or Finland.

Ms Tsai's government has pledged to undertake reforms, although as Mr Wang, the lawmaker, said, the changes in strategy and procurement will take time.

"Taiwan is competing with time, not only with the PLA," he said.

Another challenge has been filling the ranks.

Under Mr Ma, when relations improved a bit, Taiwan began to phase out mandatory conscription for all young men, which was deeply unpopular, in favour of an all-volunteer force.

The number of ground troops has since fallen to 140,000, down from 200,000 in 2005. Paying for professional soldiers eats up more of the budget.

Young men are required to complete only four months of compulsory service - Taiwan does not draft women - and then to join the reserves.

Training, however, is considered "insufficient to meet the challenges posed by the increasing threat", according to a 2017 report produced by the Rand Corp for the Pentagon.

Ms Tsai's government has sought to address that, arguing that integrating the reserves is "one of the focal points of our current military reforms."

The beach-side military exercise in Taichung last month showcased reserve forces practising alongside active-duty troops in a live-fire drill for the first time, an effort to highlight the strength of the reserves.

"We want the world to see our determination and efforts to protect our country," Ms Tsai said.

On Friday (Aug 28), she vowed not to "bow to pressure" as she unveiled a maintenance centre for the F-16 fighter jets in central Taiwan.

She said she would also work to deepen economic ties with the US, including by easing restrictions on the import of American pork and beef.

Mr Drew Thompson, a former Pentagon official overseeing China policy who is a visiting senior research fellow of public policy at the National University of Singapore, said Taiwan was taking steps in the right direction but needed to do more to break free of the traditional focus of commanders on big-ticket weapons like tanks and fighter jets.

He noted that during the recent exercises, helicopters and ground troops fired missiles normally used against armoured ground forces at targets offshore - a demonstration of adapting tactics for the most likely threat, he said.

"If Taiwan focuses on its advantages," he said, "it can beat back a Chinese invading force."

He added, "It's not nearly as dire as those people who just wash their hands and say, 'They're done.'"