Explanation: During the Franco-Prussian War, the Paris Commune arose, a radical anarchist polity centered around the city of Paris, where the working class had risen up and taken over all the levers of government.
Despite its radical leanings, it was ultimately a broad-based coalition, and so did not have absolute freedom of action even just within the context of its own internal politics - even if in its short, ~2 month lifespan it took such bold and admirable moves as remission of rents and implementation of workers' self-management over firms with absentee owners.
Si vis pacem, para bellum. Unfortunately, some people in the Commune believed that a peaceful resolution was possible - perhaps it was... but not without a much stronger push towards para bellum on the Commune's part. The French Third Republic would ultimately crush the rebellion with military force, despite being initially hesitant about its own ability to do so.
Marx, despite having some disputes with the anarchists, overall admired the Communards, but believed their essential failure was a failure to strike while the iron was hot. I would argue as many revolutions have been killed by overconfidence as underconfidence, but I also admit I'm not nearly educated enough about 1871 AD France to guess where the Communards lay from an objective standpoint.
... regardless, Marx was almost certainly right that they should've seized the fuckin' banks. An olive branch to elite powers-that-be that have made not yet made any concessions is worth its weight... that is to say, not fucking much.