The hangar at the Kunsan U.S. Air Force Base south of Seoul is filled with the signs of a war that never ended: two F-16 fighter jets being prepared to defend South Korea. Or perhaps, to attack the North.

Bombs and missiles are on the floor, about to be mounted. And there's an enormous U.S. flag, reflecting the huge American presence in the country: more than 28,000 troops.

Washington led the South through one inconclusive conflict during the Korean War in the 1950s, leaving a tense peace but no treaty. It's now the dominant player in another uncertain standoff with the North.

The Kunsan base, which has existed since before the Second World War, has seen it all. Visiting one day earlier this month — watching jets roar off the runway one after another, circling and practising manoeuvres — it certainly feels like war could break out again.

Capt. Chris (Bruiser) Brown says he's ready. He's inspecting an F-16 next to one of the reinforced concrete hangars that dot the secure area of the base. Nearby, more are being built.

Captain Chris (Bruiser) Brown in front of the F-16 fighter jet that he pilots. The U.S. has maintained a military presence in South Korea since the Korean War in the 1950s, helping to keep the peace on the Korean Peninsula. (Saša Petricic/CBC)

"If we're tasked to go north and take offensive actions, then we're prepared to do so," he says.

If U.S. President Donald Trump gives the order to "totally destroy North Korea" — as he threatened in a speech to the United Nations last week — Brown will be among the pilots sent for the mission.

I ask him if he thinks a lot about going on the attack.

After a long pause, he says. "We obviously think about it. We're constantly preparing for that to happen."

Pace of training missions picks up

U.S. forces call that "taking the fight to the North," and it's a big part of daily training missions in South Korea. The pace has picked up.

American and South Korean troops practise flying together and hitting targets out at sea with cruise missiles.

They simulate a land invasion from the North across the Demilitarized Zone – or DMZ – and respond with machine-guns, tanks and artillery.

They fire rockets to test the country's new anti-missile defence system, a series of mobile launchers developed by the U.S. and known as THAAD (Terminal High Altitude Area Defense). This is a key part of South Korea's protection.

And regularly, the U.S. calls in bombers from its regional base in Guam to fly missions further north, as a message to Pyongyang — a show of force to remind North Korea of the kind of military power Washington has at its command if Pyongyang continues to threaten the U.S. with nuclear missiles.

A technician adjusts an F-16 fighter jet in the munitions hangar of Kunsan air base. Flight manoeuvres and simulations have ramped up in the wake of U.S. President Donald Trump comments. (Saša Petricic/CBC)

The U.S. Air Force flew one of these missions last Saturday night. Eight aircraft, including B-1 Lancer bombers and F-15C fighter jets, traced their way along North Korea's east coast. They flew over international waters, going farther north than at any time since the 1990s when Pyongyang's nuclear program began.

The mission rattled North Korea. South Korean intelligence says Pyongyang has since reacted by repositioning aircraft and bolstering other defences along that coast, based on a report in South Korea's Yonhap news agency.

There was also a warning from North Korean Foreign Minister Ri Yong-ho, who said as far as Pyongyang is concerned, the United States has declared war on North Korea through a tweet from Trump last Sunday that said the regime's leaders "won't be around much longer."

As a result, the foreign minister said Pyongyang has "every right to make counter-measures, including the right to shoot down United States strategic bombers even when they are not yet inside the airspace border of our country."

'He could push us into war against our will'

The sharp exchanges, and especially the threats from Trump, have some in South Korea's National Assembly worried.

"Trump's comments are making the situation very unstable," says Kim Jong Dae, a member of the assembly and its National Defence committee.

"Trump is impulsive and unpredictable," he says, "and he could push us into war against our will. No matter how careful we are, we cannot stop him from doing something rash."

That could be catastrophic for Seoul, a city of 10 million people right near the border, and near North Korea's artillery. It's considered a prime target for a retaliatory strike from North Korea if the U.S. launches military action against Pyongyang.

U.S. President Donald Trump, left, and North Korean leader Kim Jong-un are locked in a war of words that has U.S. and South Korean troops at Kunsan on high alert. (Getty Images)

There was a glimpse of the danger recently, as Seoul held its annual drill to test civil defences during a simulated attack.

Emergency workers in red plastic suits and oversized moon helmets carried "victims" out of Seoul's subway. Others were laying on the ground pretending to suffocate.

But as the sirens wailed, many complained they didn't know where to go for safety. Critics said even if civilians made it to shelters, there wouldn't be nearly enough stockpiled food, water or medicine to sustain a long emergency.

Military expert Yang Uk, who's with the Korea Defence and Security Forum, says tens of thousands of people could die daily in conflict — or while hiding from it — because officials and emergency services aren't ready.

"They're not preparing for the North Korean nuclear attack, not preparing for the North Korean long range artillery, so right now, we are kind of defenceless," he says.

South Koreans are worried, too, much tenser than just a few months ago, when the mood toward North Korea was conciliatory.

Now, polls show rising support for building up this country's arsenals, with 70 per cent backing the reintroduction of tactical nuclear arms here. American nuclear-tipped artillery and similar weapons were withdrawn from South Korea in 1991.

Of course, all of this requires continued help and protection from United States forces, working with South Korea's own military. That explains why both seem to be on a war footing here.