Marx never advocated anything it seems. He never actually said communism was desirable, only that it would happen. The closest thing to advocating something would be the Gotha programme, but even then that's just a vague list of generic demands and not a specific law that he supported or opposed. It seems he spent most of his career getting involved in internal politics in the first international. Which, by the way, never got involved in legislature or lawmaking.
From Mark's article it does seem like he supported the franco Prussian war
When the Franco-German War broke out in 1870, Marx and Engels disagreed with followers in Germany who refused to vote in the Reichstag in favour of the war
In a 14 June 1853 letter to Engels, Marx indicated that, in the past, Jamaica had been importing new negro slaves all the time, making for a population mostly consisting of ‘newly imported barbarians’. On the contrary, the ‘present negro generation in America [represents] an indigenous product, more or less turned into Yankees, English speaking etc. and therefore becomes capable of emancipation’.85
From "The Civil War in the United States"
in the Northern States, where Negro slavery is in practice unworkable, the white working class would gradually be forced down to the level of helotry in the event of a Southern victory
He wasn't even that much against slavery, he only stated a position one time in a letter signed by 50 other people. Example
Freedom and slavery constitute an antagonism. There is no need for me to speak either of the good or of the bad aspects of freedom. As for slavery, there is no need for me to speak of its bad aspects. The only thing requiring explanation is the good side of slavery. I do not mean indirect slavery, the slavery of proletariat; I mean direct slavery, the slavery of the Blacks in Surinam, in Brazil, in the southern regions of North America.
Also, slavery was overwhelmingly unpopular in Europe, so if he opposed slavery it wouldnt be a very distinct opinion.
Was there any real, specific, non theoretical law or policy that Karl Marx had an opinion on?