Poland says that it must pursue nuclear and modern unconventional weapons.

[RiffRaff]

It's not about past scars. It's fresh intelligence indicating Russia has every intention of attacking NATO.
Biden directly warned of this in his parting speech - but of cours it's Biden, so who cares, right?

On topic, there's no reason it can't be both past scars and fresh intelligence. Poland has been invaded and occupied before, something no American has ever experienced. I don't think we can presume to understand their fear of Russia.