The difference is that US is actually putting their troops and equipment in Poland though. I also think that attack on Poland would trigger full out response by US.

Poland is a NATO state. An attack on Poland would trigger World War 3 because the whole alliance is set up to jump in.

I really doubt it. Most likely NATO would destroy any all Russian forces that enter Polish soil but not push further out past the border and escalate. No need to start WW3.

The US has explicitly refused to adopt a no first use posture.

Ditto Russia, UK, France, the other nuclear powers involved.

That being said, it’s a Rubicon all, will be reluctant to cross.

The US is the only country that used nukes.

This isn't even a remotely similar situation. The biggest difference is that last time around, we were the only people with nukes and (agree with the decision, or not) it was a war ending use of nukes, with zero chance of starting a world war, nuclear or otherwise.

Among other things, those experimental first bombs were a totally different class of weapons from today's nuclear arsenal. There were conventional bombings in WWII that killed more people than the nuclear bombings.

The UK is the only other nuclear power to have detonated nukes on the territory of another country.

Completely irrelevant.

No, in fact very relevant. Not in the way the OP probably meant it though.