you got three good answers but here’s a detailed pasta (by /u/fatpollo of reddit) relevant to the whole thread

in case anyone’s curious what a “tankie” is

Tankies don’t usually believe that Stalin or Mao “did nothing wrong”, although many do use that phrase for effect (this is the internet, remember). We believe that Stalin and Mao were committed socialists who, despite their mistakes, did much more for humanity than most of the bourgeois politicians who are typically put forward as role models (Washington? Jefferson? JFK? Jimmy Carter?), and that they haven’t been judged according to the same standard as those bourgeois politicians. People call this “whataboutism”, but the claim “Stalin was a monster” is implicitly a comparative claim meaning “Stalin was qualitatively different from and worse than e.g. Churchill,” and I think the opposite is the case. If people are going to make veiled comparisons, us tankies have the right to answer with open ones.

To defend someone from an unfair attack you don’t have to deify them, you just have to notice that they’re being unfairly attacked. This is unquestionably the case for Stalin and Mao, who have been unjustly demonized more than any other heads of state in history. Tankies understand that there is a reason for this: the Cold War, in which the US spent countless billions of dollars trying to undermine and destroy socialism, specifically Marxist-Leninist states. Many western leftists think that all this money and energy had no substantial effect on their opinions, but this seems extremely naive. We all grew up in ideological/media environments shaped profoundly by the Cold War, which is why Cold War anticommunist ideas about the Soviets being monsters are so pervasive a dogma (in the West).

The reason we “defend authoritarian dictators” is because we want to defend the accomplishments of really existing socialism, and other people’s false or exaggerated beliefs about those “dictators” almost always get in the way - it’s not tankies but normies who commit the synecdoche of reducing all of really existing socialism to Stalin and Mao. Those accomplishments include raising standards of living, achieving unprecedented income equality, massive gains in women’s rights and the position of women vis-a-vis men, scaring the West into conceding civil rights and the welfare state, defeating the Nazis, ending illiteracy, raising life expectancy, putting an end to periodic famines, inspiring and providing material aid to decolonizing movements (e.g. Vietnam, China, South Africa, Burkina Faso, Indonesia), and making greater strides in the direction of abolishing capitalism than any other society has ever made. These are the gains that are so important to insist on, against the CIA/Trotskyist/ultraleft consensus that the Soviet Union was basically an evil empire and Stalin a deranged butcher.

There are two approaches one can take to people who say “socialism = Stalin = bad”: you can try to break the first leg of the equation or the second. Trotskyists take the first option; they’ve had the blessing of the academy, foundation and CIA money for their publishing outfits, and controlled the narrative in the West for the better part of the last century. But they haven’t managed to make a successful revolution anywhere in all that time. Recently, socialism has been gaining in popularity… and so have Marxism-Leninism and support for Stalin and Mao. Thus it’s not the case that socialism can only gain ground in the West by throwing really existing socialism and socialist leaders under the bus.

The thing is, delinking socialism from Stalin also means delinking it from the Soviet Union, disavowing everything that’s been done under the name of socialism as “Stalinist”. The “socialism” that results from this procedure is defined as grassroots, bottom-up, democratic, non-bureaucratic, nonviolent, non-hierarchical… in other words, perfect. So whenever real revolutionaries (say, for example, the Naxals in India) do things imperfectly they are cast out of “socialism” and labeled “Stalinists”. This is clearly an example of respectability politics run amok. Tankies believe that this failure of solidarity, along with the utopian ideas that the revolution can win without any kind of serious conflict or without party discipline, are more significant problems for the left than is “authoritarianism” (see Engels for more on this last point). We believe that understanding the problems faced by Stalin and Mao helps us understand problems generic to socialism, that any successful socialism will have to face sooner or later. This is much more instructive and useful than just painting nicer and nicer pictures of socialism while the world gets worse and worse.

It’s extremely unconvincing to say “Sure it was horrible last time, but next time it’ll be different”. Trotskyists and ultraleftists compensate by prettying up their picture of socialism and picking more obscure (usually short-lived) experiments to uphold as the real deal. But this just gives ammunition to those who say “Socialism doesn’t work” or “Socialism is a utopian fantasy”. And lurking behind the whole conversation is Stalin, who for the average Westerner represents the unadvisability of trying to radically change the world at all. No matter how much you insist that your thing isn’t Stalinist, the specter of Stalin is still going to affect how people think about (any form of) socialism - tankies have decided that there is no getting around the problem of addressing Stalin’s legacy. That legacy, as it stands, at least in Western public opinion (they feel differently about him in other parts of the world), is largely the product of Cold War propaganda.

And shouldn’t we expect capitalists to smear socialists, especially effective socialists? Shouldn’t we expect to hear made up horror stories about really existing socialism to try and deter us from trying to overthrow our own capitalist governments? Think of how the media treats antifa. Think of WMDs in Iraq, think of how concentrated media ownership is, think of the regularity with which the CIA gets involved in Hollywood productions, think of the entirety of dirty tricks employed by the West during the Cold War (starting with the invasion of the Soviet Union immediately after the October Revolution by nearly every Western power), and then tell me they wouldn’t lie about Stalin. Robert Conquest was IRD. Gareth Jones worked for the Rockefeller Institute, the Chrysler Foundation and Standard Oil and was buddies with Heinz and Hitler. Solzhenitsyn was a virulently antisemitic fiction writer. Everything we know about the power of media and suggestion indicates that the anticommunist and anti-Stalin consensus could easily have been manufactured irrespective of the facts - couple that with an appreciation for how legitimately terrified the ruling classes of the West were by the Russian and Chinese revolutions and you have means and motive.

Anyway, the basic point is that socialist revolution is neither easy (as the Trotskyists and ultraleftists would have it) nor impossible (as the liberals and conservatives would have it), but hard. It will require dedication and sacrifice and it won’t be won in a day. Tankies are those people who think the millions of communists who fought and died for socialism in the twentieth century weren’t evil, dupes, or wasting their time, but people to whom we owe a great deal and who can still teach us a lot.

Or, to put it another way: socialism has powerful enemies. Those enemies don’t care how you feel about Marx or Makhno or Deleuze or communism in the abstract, they care about your feelings towards FARC, the Naxals, Cuba, North Korea, etc. They care about your position with respect to states and contenders-for-statehood, and how likely you are to try and emulate them. They are not worried about the molecular and the rhizomatic because they know that those things can be brought back into line by the application of force. It’s their monopoly on force that they are primarily concerned to protect. When you desert real socialism in favor of ideal socialism, the kind that never took up arms against anybody, you’re doing them a favor.