Concerning the earlier chatter in this thread about WWII and our participation in it and what might have happened had we not engaged the Axis when we did...
We fought in Europe to protect our own interests. A fragmented, diverse Europe of nation-states cannot threaten us or the rest of the world like a unified and focused Europe could, back in the day when Europe was highly militarized.
We emerged from both wars with great debt but also with great accumulated riches from allied armaments contracts and, most importantly, we emerged intact, with respect to both infrastructure and our collective political and societal identity - intact as a nation.
During WWII, our internal political leadership set down the correct priorities - defeat the most dangerous first (Germany) while holding the less-dangerous (Japanese) at-bay and making some inroads against them - and then turn all our might against the Japanese once Europe was secured.
We did not win WWII by ourselves - we were merely a decisive scale-tipping factor.
The British (and their Empire-Commonwealth), the Russians and the Chinesse, and others, all had a little something to do with it as well - both in the year or two before we jumped-in and brought ourselves online, and in the years following our entry, and including a lot of help from our allies on the technology end, as we scrambled to pull our military out of its 1920s doldrums in order to fight a 1940s war.
Had we tried to stay out of the war, both the Nazis and the Japanese would have spent the 1941-1945 (-ish) timeframe consolidating their gains and scaling-up their global reach and then they would have made their move against us from both the east and the west in a coordinated pan-Axis assault against the Americas - North and South - to eliminate the last bastions of independence and the last potential threat to their domination of the planet.
The only power capable of stopping them on the Euro-Asian supercontinent were the Russians, and things were not going well for them during the early-to-middle going, and our logistics support helped turn the tide. Without our help, the Russian turnaround might have been delayed much longer until it was too late, or never materialized at all, and the Axis would not have had a Russian Front to worry about.
With their new global reach in the 1945-and-beyond timeframe, the Axis may very well have overpowered the United States, having first overwhelmed or seduced Mexico and Canada, in order to have operational bases from which to strike at us.
But that is the stuff of Alternative History fiction writers.
One thing is certain - it would have been very, very lonely, out there all by ourselves on that thin limb-branch, without the Brits and Russians and Chinese for company.
Our victory in WWII does not make us 'the greatest country' - but it does put us solidly in the 'Greats Club' - and in the top-tier on the membership list.