This is not true. US is known for the best logistics in the world.
US power is mostly strategic, it's 'Air Based' which gives the kind of instant leverage leaders like. It's 'Aircraft Carrier Based'.
The US doesn't have that same overwhelming ability otherwise.
You can't quite an Army overseas - it takes a different kind of logistics esp. in Pacific, one the US doesn't quite have the level of sophistication.
Don't get me wrong - 'it could' but would take a while to ramp up.
US also has a large strategic-sealift force: dozens of RO/RO ships, commercial shipping agreements and prepositioned Army equipment in the Pacific and Indian Ocean.
It can move man and materiel anywhere it pleases and fast.
They are exceedingly exceptional.
But all of these systems have limits and characterizations.
US would be hard pressed to keep 500K people full supplied and engaged in Pacific. It would take a while to ramp that up.
The conflict with Iran is only 'minor' in certain respects, it's a major air campaign otherwise, involving strategic bombing on par with the Iraq War 2 - or at least the initial period.
The US is bombing them from very far away places, the amount of logistical overhead is unthinkable.
These munitions cost a fortune in part because they are purchased in lower quantities, and yes, they have to be able to do a 10x-50x instant ramp up in order to be effective (aka capacity not inventory which is the strategic key).
It takes 2 years minimum to adjust an economy in that way and the US does not have good supply chains.
It's unlikely the US could hope to fight any kind of prolonged conflict with China 'in the area of Chinese territory' for very obvious reasons: they have the supply chains, 1 billion workers, and can turn them on a dime, and don't have 'far to go' to deploy weapons.
The US doesn't even have reliable ways of getting goods and materials into the theatre and has limited quantities of everything.
That said, there are still ways to defend Taiwan and/or to make an invasion futile - but 'broad conflict' ... nope.