The US culture had nothing todo with their entry into the war.
Uh... Whatever was the then US culture either would stand for a declaration of war on Germany and Japan or not. It did. Culture had everything to do with every nation's entry (or not) into conflicts of the 20th Century.
Basically, the obsession with guns now has nothing whatsoever to do with the US involvement in WWII.
Who's said otherwise?
Cultures change, and the US culture has surely changed a great deal since the mid-20th Century. Per capita of Euro-explorers/settlers gun ownership on the territory known today as the USA has been prevalent since they immigrated to its shores. It turned out to be an essential element of how the US survived as a nation. Having and owning this personal protective gear and being willing to use it is deep rooted in US culture. My take on current US culture is that gun ownership provisions have been hijacked by paranoids (think: NRA leadership) so that responsible gun ownership has been turned on its head to allow rogue members of society to too easily get their hands on firearms.
As US culture continues to changed I think the day will come and should come when access to firearms in the USA is much tighter than it is today. I don't know when, and I don't know how, but I think the day will come, and probably the result of the whole world advancing socially and not so much just the USA.