> supply lines thousands of miles long, between two countries that are separated by 80 miles

I think this one is particularly important. IIRC, it's usually phrased something like "if the USA sends aircraft carriers across the pacific, then China has an unsinkable aircraft carrier 80 miles away: the mainland". It's a huge home turf advantage.

The USA seems to have a very low appetite for helping allies against bullies at present too. And no appetite for taking US soldier casualties.