To understand how the governments and economics of the Soviet Union (but also China) differ, one has to look at what Karl Marx said would happen. The major theme of Marxist thought on the revolution is that it would take place in a society that had reached peak capitalism. When Marx wrote his works back in the 19th century he was specifically looking at Great Britain and Germany. The idea being that capitalism would remove scarcity, but leave inequality. From that society the workers (the proletariat) would shed off their chains of the owners, landlords, and rentiers (the Bourgeoisie) and topple the system and install a new society based on "Communism."

To arrive at this hypothesis Karl Marx devoted a lot of time dealing with historical trends to prove that economics and society evolve (Hunter/Gatherer -> Tribes -> Barter System -> Feudalism -> Capitalism -> Communism). And also a lot of time discussing the ills of Capitalism to both forewarn and to also show the superiority of the new system. The biggest work on that idea was Das Kapital or Capital.

The big point is that Marx saw Communism and the "Revolution" occurring in a society in "Late Stage Capitalism."

Lenin said bollocks to that.

Russia at the time of Lenin was not fully Capitalist and definitely not "Late Stage" by any means. It was predominating an agrarian economy with some growth in industry, but far behind Germany, Britain, France, or the United States at the time. The debate was if a society/country could have a Communist Revolution before meeting the "no scarcity" portion of Capitalism, should it, and if so what would the interim look like? Lenin took the portion from Marxist teachings about the interim period right after the revolution, the "Vanguard of the Proletariat" and the "Dictatorship of the Proletariat" and stretched those theories to include protecting the revolution until the time that scarcity was eliminated, before "withering away." Coupled with the idea that Lenin had that the first revolution would spark the World Revolution that Marx had written about would mean that even if Russia was not ready per se, the rest of the world coming together would mean the deficiencies that Russia had would be removed once more industrially advanced nations joined the Revolution.

Obviously this never happened.

So to look at the Soviet Union and say that is what Marx intended is not true. No Late Stage Capitalist country had a Communist Revolution. Instead the Soviet Union was a revolution that then intended to make the conditions for Communism to happen, over time. The Soviet Union was intended, on paper, to be an interim society and government that was ushering in utopia by building the economic apparatus that would allow the next step. Leninist doctrine prescribed that the Communist Party and all its divisions was safeguarding the original October Revolution until the day came that Communism was achieved and then (in theory) wither away.

The Leninist model was adapted and used in China by Mao. It has been adapted in most countries which had a “Communist Revolution.” None of them followed the trajectory written about by Marx and none had achieved the end goal of eliminating scarcity and withering away.

Marx's communism has almost certainly never been tried in practice -- or the closest we have come to it wasn't in self-described socialist countries. The reason for this is almost vacuous: Marx simply (as you already recognized) did not specify very precisely what his communism would look like. In fact, he went further than that. He drastically ridiculed and mocked and criticized those who did try to either predict or plan future socialism(s).

The clearest idea of what Marx hoped near term steps towards a communist future to look like come from his Critique of the Gotha program. In that you find many ideas that we would describe as basic (social) democracy features. In the sense that Marx thought of them as communist rather than intermediate steps (and I don't think he did think of them as communist -- they were necessary intermediate steps for him) you would have to describe modern Sweden or Germany as adhering to "Marx's communism" more than the USSR.

He also spends a little bit more time discussing the immediate aftermath of a workers revolution (eg where the oft-misunderstood dictatorship of the proletariat term comes from), but again, those predictions regarding forming workers governments did NOT constitute his vision of communism but instead his predictions or recommendations for what the transition period from capitalism to precursors of socialism would look like.

To get an understanding of why Marx disliked predicting or describing future communism one needs to understand how mistaken Marx was about what the endeavor of prediction entails. Somehow, he believed that prediction or recommendation was simultaneously:

1. useless, as history would inevitably come to pass as it had to
2. illegitimate, because history and the proletariat were to do this job of shaping society in the future, not he as an intellectual today
3. a dangerous diversion from changing the present day

All three points are clearly incorrect -- planning would have clearly been helpful to set up real existing societies post revolution, planning does not actually entail imposition, and clearly having a solid plan is not a diversion but instead an important motivating tool that will encourage and spur action or allay fears in the present.

A relevant paper: Leopold, David. "THE STRUCTURE OF MARX AND ENGELS'CONSIDERED ACCOUNT OF UTOPIAN SOCIALISM." History of Political Thought 26.3 (2005): 443-466.

“Would Marx have agreed that the governments of China and the USSR in the 20th century accurately reflected his ideas on economics?”

Absolutely not.

One of the problems I think you'll have here is just how polarizing Marx can be; there are both wildly anti-Marx historians that probably go too far, as well as redemptive stories that gloss over both internal contradictions of Marxism and its professed practice in the Soviet Union and China. As for whether Marxist philosophy is tenable, there are very bright scholars who hold entirely opposite views. I will very briefly sketch out several major divergences of Soviet state policy from "Marxism," and offer a few considerations for what this distinction might look like.

1. If Stalin thought he was enacting Marxist policy, does it matter that Marx advocated for different policies?

Graham and Joravsky have some excellent Cold War monographs on Lysenko, an agronomist who believed that you could 'train' crops in a Lamarckian fashion. Lysenko's anti-bourgeois anti-genetic policies, maintained by both Stalin and Khrushchev, resulted in the deaths of millions of Soviet citizens. Both authors argue that the philosophy of dialectical materialism is not necessarily incommensurable with genetics, but that Stalin's view of dialectical materialism was incompatible. Is this a meaningful distinction?

The second problem here is what Marx actually advocated for. He saw this progressive view - one that would conclude without a state, so surely the Soviet Union wasn't in this sense communist - but Soviet leaders professed (as socialists) to work toward communism. Even in 1917, however, the Soviet Union lacked what Marx saw as the prerequisites (in the arc of capitalist progression) for communism.

2. What did Marx's communism actually look like?

Marx was not explicit in actually proscribing things that must be done. When he advocates for revolution - with a painfully quixotic view of what will happen as the proletarians unite - he does not provide specific things that must be done on the path to communism. Thus we can have the NEP, and vehement critiques of the NEP, and both can claim to be Marxists.

3. What would Marx have said in 1930? 1950? 1989?

I can't help but imagine Marx, by the mid-1930s (on the precipice of the Terror) would end up like Jesus before the Grand Inquisitor. However - like a tremendous number of Western philosophers - it's not unlikely Marx would have been swept up in the revolutionary fervor of 1917 and the 1920s, though he would likely be critical of Lenin's readings as well as the divergence from his arc. The construction of the bureaucratic apparatus throughout the 1920s is problematic, and by the 1930s the Soviet Union is totalitarian, not Marxist. Even by the late 1930s some Western commentators are willing to overlook the practice of the USSR for the ideals, however. But in the late Soviet Union, would a redemptive path forward be possible for Marx? Was the Soviet Union, in the 1980s, meaningfully 'socialist'? The challenge here is primarily a semantical one, but it's a pretty difficult one to overcome.

I believe the previous answers to this post have been rather deficient in giving a proper answer to your question.

Before we can even get to the USSR/Stalin/Mao, we need to discuss what Marx was actually attempting to do with his work. None of the top level answers have actually quoted Marx directly yet, so I think these are a good place to start:

In what relation do the Communists stand to the proletarians as a whole?

The Communists do not form a separate party opposed to the other working-class parties.

They have no interests separate and apart from those of the proletariat as a whole.

They do not set up any sectarian principles of their own, by which to shape and mould the proletarian movement.

The Communists are distinguished from the other working-class parties by this only: 1. In the national struggles of the proletarians of the different countries, they point out and bring to the front the common interests of the entire proletariat, independently of all nationality. 2. In the various stages of development which the struggle of the working class against the bourgeoisie has to pass through, they always and everywhere represent the interests of the movement as a whole.

The Communists, therefore, are on the one hand, practically, the most advanced and resolute section of the working-class parties of every country, that section which pushes forward all others; on the other hand, theoretically, they have over the great mass of the proletariat the advantage of clearly understanding the line of march, the conditions, and the ultimate general results of the proletarian movement.

The immediate aim of the Communists is the same as that of all other proletarian parties: formation of the proletariat into a class, overthrow of the bourgeois supremacy, conquest of political power by the proletariat.

The theoretical conclusions of the Communists are in no way based on ideas or principles that have been invented, or discovered, by this or that would-be universal reformer.

They merely express, in general terms, actual relations springing from an existing class struggle, from a historical movement going on under our very eyes. The abolition of existing property relations is not at all a distinctive feature of communism.

All property relations in the past have continually been subject to historical change consequent upon the change in historical conditions.

https://www.marxists.org/archive/marx/works/1848/communist-manifesto/ch02.htm

Communism is for us not a state of affairs which is to be established, an ideal to which reality [will] have to adjust itself. We call communism the real movement which abolishes the present state of things. The conditions of this movement result from the premises now in existence.

https://www.marxists.org/archive/marx/works/1845/german-ideology/ch01a.htm

In other words, Marx did not see communism as something that could be "tried" at all. It wasn't an ideology or a set of principles to be put into practice. He saw communist society as the inevitable end result of the development of the labor movement itself, after it successfully and completely triumphed in its struggle against the bourgeoisie.

Furthermore, Marx was a materialist, and would not believe that a society could be communist simply by virtue of its leaders being "influenced" by his writings. "Communism" was meaningless in the hands of intellectuals or politicians completely disconnected from the labor movement.

So now that we understand that, we need to ask what relationship figures like Lenin, Stalin, and Mao actually had to the global labor movement. Would Marx have seen them as legitimate representatives of the proletariat? This is a complicated and politically charged question. I don't think it would really be appropriate for the rules of the sub to give my own opinion, and giving Marx's real definitive opinion would be impossible because he didn't live to see them. But I can give a basic overview of how various prominent Marxists who did live to see them have stood on the issue.

Most self-described Marxists generally agreed that the October Revolution was a genuine proletarian revolution and that Lenin was a genuine Marxist representing the interests of the proletarian movement in Russia, and that the Revolution established what Marx would have called a "dictatorship of the proletariat". Under Lenin, the early USSR implemented the NEP, which would include "a free market and capitalism, both subject to state control". So while the USSR in this era was a dictatorship of the proletariat (rather than a dictatorship of the bourgeoisie, which is how Marx would characterize Western capitalist countries), its "mode of production" was capitalist. More succinctly, the proletariat had elevated itself to the ruling class in Russia, but the actual capitalist mode of production had still not actually been abolished in Russia. This is basically the same situation the Paris Commune was in in 1871 before it was destroyed by the French State. Marx wrote about this extensively in "The Civil War in France" if you want to see his views on it in detail, which may be helpful in evaluating the situation in Russia.

After the death of Lenin in 1924, things get more controversial. Marxist-Leninists (pro-Stalin) generally claim that the USSR remained a dictatorship of the proletariat throughout the Stalinist era, and that in the Stalinist era it also reached "socialism". What this means is difficult to parse because their explanations are usually rife with contradictions and unclear definitions. "Socialism" seems to be variously defined by Marxist-Leninists as both "the transition between capitalism and communism" and also "the lower phase of communism". How can something be both "the transition to communism" and "communism" at the same time? I'll let you try to figure that out. Marx of course, never used the word "socialism" in this way to describe any specific stage of development.

Stalin himself claimed in Economic Problems of Socialism in the USSR that the USSR had reached the Lower Phase of Communism. However, his admitted description of the USSR's economy in the book and elsewhere does not at all match the description of the Lower Phase of Communism which Marx described in Part I of Critique of the Gotha Program. Stalin describes economic categories such as "commodity production", "the law of value", and "profit" as still existing and operating in the USSR, despite the fact that Marx and Engels were clear that these were features of the capitalist mode of production (and in some cases earlier types of societies such as feudalism) and that communism would abolish these. Stalin got around this apparent contradiction by claiming that these categories were of a different type/version in the USSR than in the Western countries, e.g. the USSR had "socialist commodity production" as opposed to just normal commodity production. You can read his work if you want and compare it to Marx/Engels and decide for yourself whether that is a convincing argument.

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