I looked it up and it turns out to be from Victor Serge:
One more observation. The butcheries in Finland took place in April 1918. Up to this moment the Russian revolution had virtually everywhere displayed great leniency towards its enemies. It had not used terror. We have noted a few bloody episodes in the civil war in the south, but these were exceptional. The victorious bourgeoisie of a small nation which ranks among the most enlightened societies of Europe [11] was the first to remind the Russian proletariat that woe to the vanquished! is the first law of social war.
https://www.marxists.org/archive/serge/1930/year-one/ch06.htm