I'm sure the OP means a war on the scale of World Wars I and II, fought between many industrial nations. What made the Thirty Years War more horrendous than even wars with titles of bigger number, like the Eighty Years War and the Hundred Years War, is that it was effectively fought nonstop, as the seasons allowed, with no major breaks in fighting lasting multiple years. The 20th Century equivalent would be rather like a decades-long war beginning with what looks like World War I and ending with what looks like World War II, with fighting also filling the '20s and '30s. Not just wars of intentionally limited scope, like Vietnam or Korea, but Total War, fully involving the civilian population.
It's a bit of a dilemma, as it has to have both a good, set-in-stone ideological reasoning to continue, something to motivate an entire generation to spend its time killing eachother (the Thirty Years War had the conflict between Protestantism and Catholicism.), and it has to be before the widespread development of nuclear weapons and delivery systems (A 'hot' war between 1960 and 1990 is almost certainly impossible, almost all the land worth fighting over would be radioactive ash long before then). World War I is early enough to avoid the Bomb putting a quick end to the fighting, but was ended IOTL by one side getting exhausted and calling it off, and that was 'only' four years of fighting. World War II involved differences in ideology and threats to physical existence, especially on the Eastern Front, which would be worth fighting over for so long, but would've ended in nuclear exchange before 1969.
You would likely need to have an earlier rough equivalent to World War I, or a wave of revolutionary thinking, to encourage a shift to the ideological extremes within the Great Powers, who would then have the drive and the internal control to wage a war for so long.
I would think that America would need to have another civil war, maybe backed by the opposing sides of the wider conflict, as it would be very likely that they would eventually get dragged into the conflict and tip the scale 'prematurely' with their industry and manpower. Hard, very hard, but it may be possible, if this same theoretical wave of revolution reaches America.