This is at best debatable, it’s definitely not a forgone conclusion. A war for Taiwan would be a largely naval and aerial confrontation, and the US holds an incredibly significant qualitative lead in those two areas.
So given that the military says it would suck, I’m would not agree that it’s at best debatable. A good outcome is doubtful to start with based on war game exercises to test out this specific subject. It’s an upward fight to get it into “debatable” territory, though I’d be interested in hearing more about your reasoning.
It mostly wasn’t. The Atlantic wall was mainly a fevered Hitler dream. Parts were impressive and incredibly strong. However even strong parts were mostly taken in a few hours.
A lot of the problems were also not universal to the landings, with Omaha and Juno taking far more casualties than the other 3 beaches combined. https://edition.cnn.com/2014/06/05/opinion/lauder-eisenhower...
So on D-day it was less than predicted, but once the beachhead was established it was far from over. The total number of dead is in excess of 200,000 on the allied side alone in the battle of Normandy.
I highly doubt that retaking Taiwan after a successful invasion would be something that the general public would have the stomach for, I also highly doubt that putting the required number of troops to defend Taiwan in harms way if it came to that would be an option.
On the Chinese side there would not be any question about whether or not public opinion would be behind such a move.
Yes, Normandy was a horrific grind and taking the region was a massive and bloody undertaking. The beaches were a precursor to massive loss of life.
For another, the Allies gained significant air superiority well in advance, over the channel and over northern France, for it to happen.
Also, the German surface fleet was already mostly toast or otherwise neutralized, and even their submarines couldn't really operate in the channel with much success (see again: air superiority).
Further, Germany couldn't focus only on repelling a naval invasion in one small area—they were fighting the Allies on the Italian front already, and supporting a huge front in Russia, and had to garrison large regions of Northern Europe against possible air or naval invasion (I mean, the Allies had already tried that once in Market Garden, so it was a real possibility), not to mention to suppress occupied territories.
Plus, we're not at war and have relatively free travel. That, plus modern tech, makes spying much easier. Even if the satellites didn't spot it, a person would. We're talking about a lot of equipment, ships, and people moving around.
I would agree that the pace of war has accelerated such that some of those steps might take hours rather than months to years, but I still think the build-up would be nearly impossible to hide. You need a lot of equipment to land a force and then supply & reinforce them such that they don't become sitting ducks, and it has to be nearby from the beginning.
"But they could destroy the satellites, at least".
OK sure, but 1) they better get them all, and 2) the US would begin moving forces toward and to Taiwan within hours, if that happened—it's pointless, because they'd have already tipped their hand. It would confer no advantage for preparing a surprise invasion.
Those troops would either have to arrive on planes or in boats, and in either case they'd be heading toward an absolute hornets nest of missiles.
The most likely thing if things pop off in the western Pacific is the US fleets immediately turn tail and sail east as fast as they can to avoid ending up on the bottom of the ocean. The fleets haven't faced a modern opponent with a full armament of advanced anti-ship missiles.
The only realistic scenario where the US 'wins' in a major confrontation with China is to use nuclear weapons first, and we would still take many hits to our own cities in the process. There's no scenario where millions of Americans don't die in that war.
[EDIT] that is, I don't think you can hide a build-up for a surprise invasion of Taiwan given modern surveillance, and fairly free travel to/from China, with no existing state of war. I think any actions likely to improve China's ability to hide invasion prep, would also make it very clear what they're doing and kick things off even earlier, so, still no surprise invasion.
Now consider that Afghanistan is like 10x larger than Taiwan, much less geographically diverse, and only like 100mi from the coast of China.
It's totally possible that Taiwan could fall in a day to an overwhelming force.
War is about supply chains. Even if (big if) you managed to land several tens of thousands of Chinese regulars onto a Taiwanese beach, they are toast if you cannot resupply them. To establish these supply lines you would have to basically eliminate all enemy submarines, surface fleet, aircraft, and nearby airbases. It is a near impossible task. If the Chinese managed to do this, it would easily be the greatest military achievement in human history. It would make D-Day look like amateur hour.
This is effectively what the Taliban did. But since you struggle with that example, how about looking at Russia's take over of Crimea in 2014?
My prediction is, there will be widespread protests in Taiwan, which Chinese sleeper agents will attempt to turn violent. Then Chinese commandos storm the presidential building taking Tsai Ing-wen and her cabinet hostage, the military will be ordered to stand down. Then the subsequent riots will be crushed.
If Taiwan were just a land border neighbor to China, you might have a point. But that 100 miles of water is literally the main factor that has kept Taiwan independent since the Chinese civil war. To ignore that is downright idiotic. It is called GEO-politics for a reason. Human history is mostly shaped by geography.