Slavery was already well established long before the colonialists fought for independence. They did not introduce it afterwards.
That dreadful word "nuke / nuking" cheapens one of the nastiest of all human inventions. Those two nuclear bombs did finally force the Japanese to surrender, ending WW2 in Asia a year after it had ended in Europe, but also showed the world just how terrible nuclear bombs of any sort are. Those dropped on Hiroshima and Nagasaki were fission weapons of "only" <20kt power, too: the weapons developed later by the USA and USSR are considerably more powerful, and hydrogen-fusion bombs vastly more so.
Also, Japan was already fighting in the late 1930s by trying to invade China.
If America had not become independent it would be a hotch-potch of British, Irish, German, Dutch, Spanish, etc. territories perhaps subsequently becoming minor nations of their own; and there is no guarantee the natives (a proper word, referring to generational birth-place as its etymology shows) would have been treated any better.
Although slavery may have ended earlier: Britain had been an active slave-trader but was the first country to ban it and as a major cottom importer, applied commercial pressure to help end it in the young USA.
