The threat (?) of China

I doubt China would ever deliberately just "attack the US".  There's just nothing in it for them.

However, the odds that China ends up in a REGIONAL war that ends up with them fighting one of our allies (Taiwan, Japan, even India), and us getting sucked in are certainly uncomfortably greater than zero.

Then you have a Japan 1940 situation: an untested but arrogant and growing military power, clearly a regional power, trying to shoulder its way amongst other regional powers, but intelligent enough to understand that ultimately the guarantor of the status quo is the US.

Then what?

Then China's motivation becomes a game of poker; to what level must it raise the geopolitical ante, such that the US will walk away from the game?

THIS is the critical, one might say almost existential, question of the next 20-30 years between the US and China.  I fear it won't take longer to resolve than that.  Perhaps much less.

The calculus here is complex and not encouraging.
China is a burgeoning economic, demographic, exuberant 'growing' power.  The US is the pre-eminent military power bar none, but its economic system is a sham, its leaders (on both sides of the political fence) feeble, incompetent, and utterly self-interested.  The US public is lazy, apathetic, ignorant, and cheerfully distracted by TMZ.

On the other side, of course are a few critical questions:
- How much of China's economic clamour is real, and how much is Potemkin smoke and mirrors?
- China has severe demographic issues regarding gender balance, youth, and the growing bitter gulf between the haves and have-nots both within China and compared to the rest of the world.  These are only getting worse, and are amplified by a government that seemingly likes to toy with jingoism - neglecting the lessons of history that suggest in the long term it's something nearly as dangerous to the sponsor.
- Russia is a dangerous wildcard, remaining a revanchist proto-Soviet state under Putin.  Wide, empty, resource-rich spaces of Eastern Russia make this an extraordinarily sensitive spot for a Russian military already overstretched.  Russian ego-investment in their far-eastern districts is also not insignificant.  Their participation in any conflict would be opportunistic, aggressive, and extremely brittle.
- India, likewise, is extremely wary of a growing Chinese state lurking not-far-enough-away on the other side of the Himalayas.  The 'great game' West of Mongolia has been quiet since WW2, but it's been in play since the late 1800s and certainly isn't resolved yet.

All of these are a subtle confluence of varying factors; the reality of each doesn't even matter so much as the Chinese PERCEPTION of reality.  And this introduces what I think is the absolutely greatest wildcard: Chinese understanding (or lack thereof) of the US.

The US population is largely lazy, overweight, and apathetic.  Would an actual attack on the US be enough to rouse them from their torpor?  I honestly don't know, but I'm absolutely CERTAIN China has no clue.

What I fear is the habitual misunderstanding - and critically, underestimation - of US culture by others.  As much as any "foreigner" can fail to understand any culture (the US trying to understand China, for example) the US seems to regularly and particularly baffle outsiders, even people from relatively close cultural analogues as Germany, or France, much less China.

What I fear specifically is that the Chinese mandarins will, wearing the same blinders that Chinese have had for at least 2000 years, dismiss the US's potential response.  Like the belief in some quarters pre-GulfWar2 that the US "won't put up with casualties, ergo they won't get involved" - this both right and desperately, critically wrong.  The US public does habitually have an intolerance for suffering, no question.  But the lesson of history is that when enraged, we're capable of rationalizing and accepting staggering violence in the pursuit of "justice".  Even the relatively showy but strategically trivial attack of 9/11 motivated us to deploy our ground military for 10+ years WITH NO STRATEGIC GOAL.  Heck, we didn't even really use our air force or navy.

If there was an actual, direct attack on US citizens, I genuinely fear the response of the US public, and I'm not sure China does.  And in that inequality lies a terrible, terrible danger.