The point is that without US involvement in those efforts arguably it's highly unlikely that Germany would have been defeated as it was, not to mention Japan's then aggressive imperialism.
Sure, the US played an important part but Germany was pretty much doomed to fail after their disastrous Russia campaign and the Russian war machine slowly moved into gear, The Russians paid the heaviest price of all in terms of human sacrifice of course.
If true that US involvement made the difference in outcomes then the US culture leading to that involvement was an essential element leading to defeat of tyrants of the 20th Century.
The US culture had nothing todo with their entry into the war. If anything the isolationist attitudes kept them out of the war longer than they should have. It was only being attacked that caused them to take part (by both Japan and Germany).
Gun ownership is part of that culture.
Yes, but that is misleading and a causal leap. It would be as valid to claim that slavery was part of the culture and so helped to win WWII. Obviously false but just as valid a claim.
As said already, whether that citizenry's culture would have developed as it did to perform as it did in the 20th Century without its precursors holding very personal views about gun ownership I don't know.
Well the rest of the world managed to fight without having a gun culture (which the US didn't have back then to anywhere near the same extent anyway - that started in the 70's). Plus the gun-culture you claim didn't encourage the US to enter the war as you claim.
Basically, the obsession with guns now has nothing whatsoever to do with the US involvement in WWII.