By "successful" I mean to about the level of the Arab Spring--that is, a few, perhaps four or five, countries of reasonably large European importance where the revolutionaries create lasting change, or otherwise achieve more or less the sum of their goals, possibly after a lengthy civil war. OTL, the only permanent changes were in marginal countries (Switzerland, Denmark) with the exception of France (where the Second Empire stemmed from the creation of the Second Republic in 1848), at least to my knowledge.
So, what would it take? What would the side effects be?