China can bomb TSMC, no doubt.

But landing troops (they can't march, it's an island, and the difference between amphibious and land based operations matters a lot) would be extremely difficult, and not obviously in the PLA's favor. See, for example:

https://foreignpolicy.com/2018/09/25/taiwan-can-win-a-war-wi...

China's sheer scale means there aren't too many military shortcomings it can't address on a roughly 10-year timeframe.

The larger point is that it'd be way too messy for them to even try. Basically on the scale of US/SK invading NK.

Besides that significant causalities and economic damage to both sides, which of course they'd eventually win, but it'd be a geopolitical disaster that won't end with the quelling of the armed forces.

China has far more to gain not going this route.

I’d imagine China would more likely begin blockading Taiwan, at least until the US and the west responded. Given the strength of US naval power it’ll be a while before China consider even that.

If China mearly wanted to destroy Taiwan's civilian society, they could do it. But to take out the defences and make an amphebious landing across a large and dangerous sea is extremely hard. Perhaps possibly only with cyber-warefare to disable the defenses.

This is why the US just struck a deal to have TSMC build a fab in Arizona. It's absolutely critical to have a backup plan.

Isnt its planned output tiny compared to the production volumes in Taiwan?

Having the foundations of a fab makes it a bit easier to expand production in the future compared to setting up the whole shop at once if needed.

Yes. Everything points to it being done for a DoD order.

My understanding is that the actual machineries and materials necessary to make and run fabs are made in US, Japan and Europe, not in China or Taiwan. For example photo-lithography machines are made by the like of ASML (Netherlands), Nikon (Japan) and Canon (Japan).

Although it would assuredly take some time to ramp up, TSMC should be able to spawn fabs outside of Taiwan, out of CCP reach. They are already building one in the US, albeit with a small output.

Their whole supply chain is going to grind to a halt in that scenario even if TSMC's fabs were somewhere else. China would at minimum get sanctioned and there'd be component and raw material shortages for a while.

The United Nations would issue a strongly-worded letter. Markets would fluctuate for a week. Then everyone outside of Taiwan will pretend that nothing happened.

Could this lead to a large chip fab being built in the US? Or do they just hop to Japan / South Korea?

If we're in a scenario where Taiwan is getting bombed or invaded, South Korea and even Japan aren't exactly safe options either.

TSMC has fabs in mainland China as well. China is a friend of capitalism.

According to Wikipedia, TSMC's 9th largest fab is based in China. Their 1st - 8th largest fabs are based in Taiwan.