Why is the US Navy practising for war with China?

You don't get invited out on a US nuclear aircraft carrier all that often, and after writing this I might not get invited back for a while.
On the flight deck of the USS George Washington the noise is like nothing I've ever experienced. A few feet from where I am standing, 11 F/A-18 Super Hornets are lining up to be launched.
The first one is hooked on to the catapult; there is a massive crescendo as its engines roar to full re-heat. Then, in a cloud of white steam, the 15-tonne jet is thrown down the deck and off the end of the ship like a toy.
Seconds later, the deck crew in their multi-coloured smocks are calmly lining up the next one.

Watching the US Navy close up like this, it is hard not to be slightly awed. No other navy in the world has quite the same toys, or shows them off with the same easy charm.
But as I stand on the deck filming my report on how "the US is practising for war with China", I can see my host from the Navy public affairs office wincing.
You get used to hearing the PR rhetoric: the US Navy "is not practising for war with any specific country". But the US Navy has not assembled two whole carrier battle groups and 200 aircraft off the coast of Guam for a jolly, either. This is about practising what the Pentagon now calls "Air Sea Battle".
It is a concept first put forward in 2009, and it is specifically designed to counter the rising threat from China.
A few minutes later I am standing on the bridge of the George Washington with Rear Adm Mark Montgomery, the commander of Carrier Strike Group Five. The forces under his command are practising for what he calls an "anti-access, area denial" scenario.

"When we talk about our capabilities," he says, "we are talking about our capabilities to operate in unrestricted way in the waters of our choice".
"As some countries have been developing increasingly complex anti-access weapons, we have to develop our tactics, techniques and procedures to continue to operate in an unfettered manner."
Rear Adm Montgomery won't discuss the specifics of the exercise. But his ships and aircraft face an increasingly complex web of threats, from beneath the water, from air, land, from cyberspace and from space.
"It's generally understood that some countries have the ability to remove satellites or to limit satellite communications," he says, "so we have to practise working in a communications-denied environment."
China's People's Liberation Army Navy is still no match for the US Navy, and won't be for a very long time. Instead, China has been developing other weapons designed to keep America's precious carriers far away from China's shores.
These include new quieter submarines, long-range hypersonic anti-ship missiles and, perhaps most worrying, very accurate medium range ballistic missiles that have been dubbed "carrier killers".
As if on cue, an alarm bell starts ringing. A voice comes on the public address system:
"This is a drill, this is a drill! Black smoke, black smoke!"

The George Washington is under simulated attack. Part of the ship is reported to be on fire. Teams rush to contain the damage.
For the last 10 years, China's most important, and oft-repeated, political slogan has been "peaceful rise". It is designed to reassure Beijing's neighbours its growing military might is no threat.
But since President Xi Jinping came to power last year, there has been a distinct change. China is now asserting claims well beyond its own coastline.
Its ships are aggressively patrolling the Senkaku, or Diaoyu, islands in the East China Sea, long controlled by Japan. It is spending billions building new islands in the South China Sea.