Canada

• an offensive war is much harder than a defensive one.

Agreed

• America would have to maintain gigantic transoceanic supply lines.

True, but America already maintains these, even without ongoing conflicts. If they moved carrier groups from the Pacific, even easier.

• America has a low tolerance for casualties, especially in foreign lands.

I don't think so. They seem pretty okay with casualties in every war since Vietnam. Compounded with the likelihood most casualties wouldn't be extracted to the US until the end of a major conflict, it gets even easier.

• Such an invasion would cause huge social unrest at home. Presumably such an invasion would be instigated by trump who would then be compelled to deploy the military at home as well.

Sadly, this could legally work. The USA military is beholden to the president over anyone else. So long as an action isn't unconstitutional, it's legal if it comes from POTUS.

i don’t like to give the US credit but they have indisputably the most advanced global military logistics of any country in history.

The Americans can drop 100,000 troops anywhere in the world, have complete supply lines established in 24 hours, and have immediate air and sea superiority. If we are talking Europe, they already have a significant military presence in place.

The US military is immense and insanely equipped. The hundreds of billions of stuff sent to Ukraine is mostly old stuff that would have aged out anyway. They have not even touched anything they would use themselves.

I know “Britannia rules the Sea” but honestly, look how many aircraft carriers the US has. No contest. Also, how many cruise missiles do they have to disable military infrastructure.

US intelligence is similarly sprawling and deep pocketed. Their intelligence failures are almost always political. In an actual military led conflict, this puts them at an extreme advantage.

The Americans have also shown extremely effective use of patriotism and propaganda. It is unlikely that resistance at home would emerge quickly enough to prevent a successful invasion.

As of late, they have also demonstrated the ability to absolutely give the middle finger to their allies and international diplomatic bodies.

Where the Americans suck is in holding territory and in maintaining a focused agenda. A war with the UK and the US would probably result in regime change on both sides ( one through force and one through popularity ). However, in the UK, they would end up otherwise leaving pretty much the entire bureaucracy in place and, after wasting craploads of money, would probably pull out and go home. This is where all that resistance at home and abroad stuff would overpower them.

At the end of the day, Britain would be largely unchanged after the US pulled back out. The US would have gained almost nothing. If they were lucky, things would look much as they do now.

In fact, now that I have walked through it, I guess the answer is that I would certainly not fight against the US. It would be for nothing. Better to just wait.

In fact, the UK would be better off not fighting too hard and then just negotiating themselves back into power afterwards. I mean, the PM would have to change but that is hardly a big deal.

I am wondering what policy the UK would have to abandon. I mean, something must have caused all this in the first place.