I don't think a gernuinely peaceful twentiewth century is possible without changes so majoer the idea becomes meaningless (colonisation by a surviving Song China might lead to a peaceful period in 1900-2000 AD, but I guess that's not what you mean). European culture had embreaced war to too great an extent.
That said, I think it is possible to have a much less disastrously warlike twentieth century. IMO your best bet is taking Prussia down a peg or two. United Germany was a massive disturbance in the force smack in the middle of the continent. If that doesn't happen, France keeps its sphere of influence in Italy and Southwestern Germany, and Prussia stays hegemon of Northwestern Germany, you have a much better chance of averting the seminal disaster of WWI. Next, throw a war into the mix that is major by previous standards, though smaller than WWI. Have the horrified powers of Europe see two of their number bankrupt themselves in a fruitless slogging mastch. Governments will be much less willing to risk economic and demographic armageddon (everybody was horrified by WWI, but nobody quite knew how to end it).
An additional bonus would be to maintain the primary confrontation line between Britain and France. The two can't really touch each other, so even in the event of a war, it wouldn't be as destructive asd the Germany/France slogging matches.