How a US aircraft carrier is part of a high-stakes stand-off in the South China Sea
There are few greater displays of US military might than its aircraft carriers - when they move, the world pays attention. They are huge floating cities of sorts.
The US has 11 and they are still the best in the world.
The USS Nimitz is one of them. It's a warren of cabins and control rooms, beneath an enormous flight deck. It can carry around 5,000 military personnel and as many as 7,000 when you include the accompanying "strike force" of war ships and jets.
We went aboard when the ship was docked temporarily in South Korea but had just completed a deployment in the South China Sea.
Deployments like this are routine, but they are undertaken in the knowledge they will be noticed by one power in particular, and that power is China, the US' increasingly assertive competitor.
Lt Ben Bushong, who served in the US navy for seven years - much of it as a helicopter pilot - showed us around.
He and his colleagues know things are tense, as they have been for some time.
"The big picture is helping keep a free and open Indo-pacific," he said.
"I would just say we're always ready and we're always training, so if we ever get called we're ready to respond."
And rarely has being ready felt more pressing.
The US' face-off with China has multiple fronts and almost all are becoming more fraught.
Just a few weeks ago, China's new foreign minister said that if the US does not "hit the breaks" on what it sees as provocations then "conflict will surely follow".
China has increasingly been making a point of flexing its muscles in the seas that surround it.
For many years it has been accused of militarising islands in the contested South China Sea and engaging in illegal fishing, but recently it has also been sailing and flying much closer to the self-governing island of Taiwan.
There have also been more specific incidents of concern - just last week, China said it chased away a US vessel that had "illegally" entered waters around the contested Paracel Islands, a claim the US denied.
And then there was declassified footage shot by a US jet of a Chinese fighter flying just feet away from it, in a move the US has described as aggressive and dangerous.
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The reality is that China can now afford to flex these muscles because it has rapidly developed the largest navy in the world, increasing 10-fold in just the last 20 years.
But it is the reason that US presence here is so important, according to the Rear Admiral Christopher Sweeney who leads the Nimitz's strike fleet.
"First, we are going to sail operate and fly wherever international law allows," he said.
"And my message to the PRC (People's Republic of China) would be that we're resolute in that, we're not going to be bullied, we're not going to be coerced and we are going to stay here and fly and operate in international norms."
China sees the US efforts here as part of a broader effort to contain it. Fundamentally, it sees this region as its back yard.
But in answer to that allegation the admiral was resolute.
"We do not seek to contain China, we do not seek conflict with China, we seek to set international norm that we all prosper from".
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If fighting did break out in this region, it would most likely be over the island of Taiwan, the self governing democracy China sees as its own.
China has not ruled out taking the island by force and deterring this is one of the US' key objectives.
In the meantime, China's increasing assertiveness is pushing many other Asian nations closer to the US and each other.
The show of allegiance with South Korea was front and centre - the two are undertaking joint drills this week and will work with Japan too next month.
While much of the tension between the US and China is invisible and covert, this is a region that is increasingly fractured and there is a high-stakes stand-off playing out.