It's the kind of scenario the world hopes to never find itself in, but one that Japan and the United States rehearse for anyway.
After being called in to bolster the defences of a key ally, US marines have found themselves in a precarious situation.
In this scenario, the troops have landed after a hostile force, codenamed Country Red, threatened to invade the island nation of Japan after a build-up of enemy troops.
But now the marines are in imminent danger after the enemy sent in special forces behind US positions.
Fortunately, it's just a drill, part of the biggest bilateral military exercises held on Japanese soil, on the northern island of Hokkaido.
For the marines, however, the fictitious scenario is a perfect opportunity to practise some skills they may need in a real-life operation.
While Japan was obliged to "forever" renounce warfare and only maintain an army for self-defence after World War II, the country finds itself in an increasingly volatile neighbourhood.
It now faces challenges on three strategic fronts: a rising China, an increasingly aggressive North Korea, and Russia's invasion of Ukraine.
A missile with a message for Putin
Anti-tank missile man Corporal Chance Brown rushed to a high vantage point, armed with one of the most sophisticated anti-tank weapons: A Javelin missile launcher.
Within moments of placing the launcher on his shoulder, he locked onto his target, and the missile was hurtled high into the sky before changing course to strike the top of the target.
The explosion prompted cheers from his fellow marines.
Despite his extensive training, this was the first time Corporal Chance Brown had actually fired the Javelin with a live round.
If he was feeling any pressure, he wasn't showing it.
"That felt really good," he said afterwards.
The FGM-148 Javelin is a smart anti-tank weapon system that rose to prominence in the early weeks of the Ukraine war, as small, mobile units of Ukrainian defenders used it to wreak havoc on advancing Russian tanks.
It proved so effective against the invaders that it earned the reputation as "Putin's nightmare" and was converted into an internet meme, "Saint Javelin".
The Javelin has what's called a "fire-and-forget" system, which allows the shooter to run away to safety after launching the missile, which is locked on with infrared guidance.
This advanced weapon not only has attracted a lot of attention this year but so, too, has its price tag: About $US184,000 for just the missile alone.
"It makes you feel more special," Corporal Brown said.
"This is my weapon. I know everything about it. I know how to deploy it."
Japan's increasingly volatile region
The military exercises — called Resolute Dragon — come at a crucial time for Japan's Self-Defence Forces, which are undergoing some of the biggest structural rethinks in a generation.
Japan has long been wary of an assertive China to the south.
However, Russia's invasion of Ukraine, and North Korea's recent flurry of ballistic rocket launches — and its commitment to building a nuclear arsenal — has drastically elevated Japan's desire for stronger deterrence and a much bigger military budget to boot.
"Japanese policymakers emphasise the incredibly insecure position [that] Japan finds itself in," an expert in Japan-Russia relations at Temple University, James Brown, said.
"Russia to the north, North Korea to the west and China as well."
This year's Resolute Dragon was the first time the Javelin had been fired for Japanese forces on home soil, demonstrating the weapon's new-found importance and the cooperation between the two sides.
The commanding officer of the US Marines' 3rd Battalion, Lieutenant Colonel Felix Guerra III, admitted firing Javelins at military exercises was "rare", but said it was important for Japan to see the weapon "in action" on "home turf".
"It's a good symbol for our partners, our allies here, the Japanese, to see that it's a valuable asset," he said.
"It just shows our commitment, that — if we ever had to help them out in a mission — we're going to bring our nice toys with us, our expensive Javelins, and other tools."
Mr Brown said the decision to bring out the Javelin sent a message.
"It's primarily an anti-tank weapon," he said.
"The chances of Russia launching an invasion of Hokkaido are very minimal.
"The decision to use this [is] primarily symbolic rather than practical: reminding the Russian side just how potent the weapon has been in Ukraine."
Russia has since filed a protest with Japan's embassy in Moscow, labelling the country a threat to Russia's far east.
Locals caught in midst of escalating tensions fear the future
The United States and Japan have a long history of holding bilateral military exercises across the island country.
But Resolute Dragon — now in its second year — is the largest and most-sophisticated held on Japanese soil, on the northern island of Hokkaido, where there's plenty of space for war games.
Lieutenant Colonel Felix Guerra III said he expected more complex weaponry to be used in future exercises.
"It raises the bar on our inter-operability," he said.
But the escalating military tensions coinciding with the military exercises have left many residents in Hokkaido feeling uneasy.
"I think there are many worrying things happening, compared to before," resident Hideo Fukuzawa said.
"It's distressing and unsettling for the local residents."
The 82-year-old has a unique connection to recent conflicts.
Earlier this month, he and other locals were urged to seek immediate shelter after North Korea fired a ballistic missile over northern Japan, something that hasn't happened since 2017.
The war in Ukraine has reminded him about his own childhood trauma, when the Soviet Union invaded his home in the final days of World War II, seizing a string of islands that were never returned.
Earlier this month, the Ukrainian parliament passed a resolution recognising Japan's claim to the islands.
"[The Russians] threatened us with a gun. I was only five years old, but I still clearly remember that, as a child, I felt the fear that I might get killed," he said.
"I saw a child in Ukraine [on the news] … who was shedding big tears and crying, telling his mother he didn't want to die.
"I felt it was just like me, 77 years ago. It broke my heart."
He said he worried the increasing closeness to the US might make Japan a target.
But he also appreciated the need for Japan's Self-Defence Forces to be as prepared as possible.
"When Japan shows it's being close to the US, Russia and China become perverse and pick on Japan," he said.
"I wish it would stop."
