A battle for Taipei could hinge on defending this river. Here's how Taiwan's forces are readying for that fight.

• While concerns about Taiwan's defense often center on beaches, one of its major vulnerabilities is a river.
• The Tamsui River cuts into the heart of Taipei, where its government and military headquarters.
• Taiwanese forces drilled earlier this month to defend it in multiple urban scenarios.

TAIPEI — In the dead of night, scores of masked military police moved a medley of defensive gear beneath flickering traffic lights and store signs. Hemmed in by rows of shophouses, they laid metal barricades, land mines, and sandbags along an intersection just a mile from Taiwan's presidential office.

Behind them, Clouded Leopard infantry fighting vehicles prepared to roll out and open fire on incoming enemy forces.

This flurry of activity along a critical approach into the capital came toward the end of Taiwan's 10-day annual Han Kuang military exercise. In this scenario, the simulatedfight had made its way into central Taipei. The blockaded roads led to the Guangfu Bridge, a four-lane crossing that spans the river dividing New Taipei City and the capital proper.

The Tamsui River, snaking through northwest Taiwan, was at the heart of many of this year's Han Kuang drills. Although concerns about an attack by Beijing often center on a massive, D-Day-style landing across the Taiwan Strait, Taiwanese forces have spent considerable time practicing their defense of the Tamsui.

The strategic waterway, fed by some of the island's largest tributaries, converges in the capital and then spills into the strait, where the Chinese province of Fujian lies some 100 miles across the water.

That could provide Beijing with a direct route into central Taipei, with the presidential office — a major invasion objective — positioned less than a mile from the river.

Though the Tamsui gets far less public discussion than Taiwan's beaches, it's a "real geographic vulnerability" that Taiwanese forces have been training to defend for years, said Tristan Tang, a nonresident fellow at the US think tank National Bureau of Asian Research.

"China has Zubr-class air-cushion landing craft that can move at more than 50 knots, and PLA air assault troops could also come in by helicopter," he told Business Insider. "So from a capability standpoint, the PLA does have ways to potentially use the Tamsui River corridor to get into Taipei."

That makes the Tamsui part of the "iron triangle," a trio of features centered on the estuary that are considered vital to Taipei's defense. Next to the river's mouth are Bali Beach, one of the more plausible landing areas near the capital, and Taipei Port, which would allow an invading force to offload troops and supplies quickly.

Crippling Taiwan from the river

Preventing a "surprise thrust" by way of this river is key to the island's defense, said Tang, who designeda high-profile war game focused on a Chinese invasion of Taiwan.

The Han Kuang exercise featured several drills to prepare the island's forces for that possibility. During one drill held early in the morning on the third day at the mouth of the river, which is up to 4,500 feet wide at the opening, Taiwanese divers and army soldiers placed dozens of pontoons strapped with explosive barrels while CM11 Brave Tiger tanks kept watch.

Delaying measures such as this could prove crucial for Taiwan. A common view among American and Taiwanese analysts is that the island's defense depends heavily on its ability to hold out long enough for outside forces to intervene.

Lin Ping-yu, a former Taiwanese special forces paratrooper and now a city councilor for New Taipei City, said defending the river is aimed at preventing decapitation strikes or surprise attacks by the People's Liberation Army.

"China has studied decapitation scenarios for many years," Lin, an advocate for Taiwanese defense issues, told Business Insider, arguing that "recent incidents of Chinese nationals entering Taiwan illegally underscore why the Tamsui River is the front line against such strikes."

Taiwan's coast guard said earlier this month that it was holding two Chinese nationals suspected of infiltrating the island with a rubber boat, potentially exposing a security gap.

It's clear Taipei is thinking about the risks. On the first evening of the Han Kuang, Taiwanese President Lai Ching-te and members of his staff responded to a mock decapitation strike, evacuating from their offices in an armored vehicle to a fortified command center.

Tang said Taiwan's emphasis on the Tamsui is especially evident in how it has placed the Guandu Area Command, one of the key army formations defending the capital area, along the river. Its marine corps and military police, responsible for guarding Taiwan's leaders, have bolstered their presence in the area in recent years.

"The Taiwan military understands this and has been preparing for it for a long time," he said.

Taiwan's new strategy

Drills from the last two years have seen Taiwan moving troops from multiple brigades in the south to reinforce the island's capital area, Tang noted.

Those added forces, he said, would make "a successful PLA surprise attack against northern Taiwan more difficult" while filling the capital region with more forces to deny Beijing's even if China seizes other regions of the island.

The Tamsui plays another essentialrole here, too. Chinese forces seeking to enter the reinforced capital from the south would need to cross the river. During the Han Kuang drills, Taiwan's troops practiced that defense on a local bridge infrastructure.

At the Danjiang Bridge, a new, 3,020-foot suspension bridge opened in May, engineering troops and volunteers set up a three-layer defensive barrier of Hesco barriers, barbed wire, and Czech hedgehogs, a kind of pronged anti-vehicle defense.

"The Han Kuang exercises also show how seriously the military takes this scenario, including the preparation of actual defensive positions and field fortifications," Tang said.

Taiwan's defense ministry did not respond to queries from Business Insider aboutthe defense of the Tamsui River.

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