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

He also argued against environmentalism/malthusianism, although that still isnt a law. He might also have been critical of immigration. His slavery position was... odd

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?

  • Does it answer to your question? en.wikipedia.org/wiki/The_Civil_War_in_France
    – fedor
    Jul 30 '20 at 23:16
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    This Wikipedia article contains some wrong information. The only time Marx mentioned "dictatorship of proletariat" is in his letter to Weidemeyer, google.com/… which was published long after Marx's death.
    – user33085
    Jul 30 '20 at 23:33
  • His writing on the Paris commune is just as indecisive
    – user33390
    Jul 30 '20 at 23:52
  • This is also not quite true. Marx's idea was that the Paris commune failed precisely because the organizers did not fight decisively enough. That was later explained in detail by Lenin.
    – user33085
    Jul 31 '20 at 1:16
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    But that's not the same as supporting or opposing it
    – user33390
    Jul 31 '20 at 1:21

To pick just one intesting if minor example, Marx clearly supported the abolition of slavery. For example, he signed a letter praising Abraham Lincoln and declaring "the triumphant war cry of your re-election is Death to Slavery". You can find many articles online discussing Marx's writings regarding slavery and the US Civil War.

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    But that seems to be for pro-white reasons
    – user33390
    Jul 31 '20 at 1:23
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    Yes, pro-white. As most 19th century philosophers, Marx was a racist, pure.uva.nl/ws/files/44504342/…
    – user33085
    Jul 31 '20 at 2:01
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    It wasn't even that consistent of a position, he only opposed slavery 1 time and he was writing on behalf of other people
    – user33390
    Jul 31 '20 at 5:49
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    @JohnBalvin He wrote an entire collection of articles about it. He also argues against slavery and for black emancipation in Das Kapital. He never argued for slavery, so saying that it wasn't a consistent position doesn't seem correct.
    – tim
    Jul 31 '20 at 9:50
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    Marx opposed slavery on multiple occasions, and Engels used a comparison of capitalism to a new form of slavery precisely as a rhetorical device against capitalism. He ridiculed Proudhon, writing that only with Proudhon's logical leaps could one come to the conclusion that slavery had some good components. It's also unclear what is meant by "pro-white" in the case of opposing slavery, @MarkSapir's paper does not back that interpretation up.
    – q3d
    Jul 31 '20 at 9:50

Marx of course advocated communism and did everything in his power to justify it and bring it closer. For example his role in the first International was very significant, This article says, in particular,

"Although he was neither its founder nor its head, he soon became its leading spirit."

What is true, though, Marx never promoted what is called "communism" now. Even Lenin would be an "anti-Marx" in his view. He also hated liberals, trade unionists and so on. His main ideological enemy was Lassalle, the founder of what is now known as "democratic socialism".

  • That article says he just got involved in internal politics of the organization and destroyed it
    – user33390
    Jul 30 '20 at 23:54