Karl Marx said, 'The history of all societies is the history of class struggle,' or so they say.
Literal
Karl Marx says-[quotative-曰く] 'all societies [gen-の] history [topic-は] class-struggle [gen-の] history [copula-である]' [hearsay-だそうです].
曰く (いわく) is the classical quotative 'says / said' — used specifically for attributing quotes to notable figures. 階級闘争 'class struggle' is a Marxist technical compound. である is the formal/written copula. だそうです is polite hearsay — the speaker is reporting a quote they've heard. The layered attribution (Marx says X, I hear) creates epistemic distance.