Could WWI and WWII be considered a second "Thirty Years War"? What allows historians to combine the various religious wars in the Holy Roman Empire at the beginning of the 17th century into the Thirty Years War, but prevents them from combining WWI and WWII into a similar grouping, despite both conflicts being fought over a similar scope of time, and each maintaining strong ties from the end of the first war in the "period" to the motivations for the second?