Like allowing multiple political parties, full freedom of speech and assembly, abolishing the police, ownership of weapons, direct democracy etc.
The common justification is that they were in a dire situation where allowing too much freedom would allow counterrevolutionaries and foreign imperialists to sabotage and destroy them. I find this unconvincing, to what extent is security better than freedom? To what extent can the current leadership be trusted to "protect the revolution" than possible others better suited who couldnt take power?
Even then, why did the Soviet Union and other communist countries not democratize after WW2 when they arguably established sovereignty with their nuclear weapons?
Just as the capitalist ruling class preferred fascism to losing their power to communists, it seems the Marxist-Leninist rulers preferred capitalism to a more democratic form of socialism.
We see this happen now in Cuba, the last bastion of Marxism-Leninism, where the ruling class has been gradually introducing privatization and market reforms rather than allowing things like open elections, freedom of speech etc. Under capitalism, they can still rule.
This whole "you need to have multiple political parties who are in constant conflict with eachother in order to have democratic freedom" is an extremely weird western-centric point of view. Does a communist country need capitalist opposition parties? Does a communist country need multiple communist parties? Why would it matter how many political parties there are if the country is otherwise democratic?
A one-party state is the equivalent to a no-party state - yet the emotional response to such a thing is completely different. It begs the question why and I think the answer is extremely obvious to anyone with any sense of historical context. Everyone's opinion starts and ends with "one-party" as if this is some how the only measure of freedom, since that is literally the only extra choice there is in a liberal democracy. The average person in the west has virtually no idea what socialism or communism means other than "one-party dictatorship" because that's literally all the western propaganda powers had to do in order to convince the populace that this is what separated us from them, completely disregarding the fact that dictatorships all over the world were and are still allied to the west and have been propped up by the west. The only thing that mattered to the ruling class in the west was that their dictators were "capitalist" and that was good enough for the average person who imagined that term to be interchangeable with "free".
As far as "nukes establishing their sovereignty" you are correct... which is why there was never a military invasion of the nuclear powers. The way the CIA and other intelligence agencies would work to destabilize the communist countries would be political actions, which would be made incredibly easy in a country that decided to liberalize. Look at what happened to the USSR after the cold war had already died down and tell me they should have done it earlier. None of the countries involved have recovered and every single country is worse off than when they were in the USSR.
No liberal democracy would survive extended economic warfare with the united states much less the entire western world for very long at all. It is not a coincidence that the only countries still standing against them have "strongmen" type governments, because all the "free" countries that stood against them fell extremely fast. The United States didnt accidentally become the world's dominant economic power, they've been forcing their will onto others for a hundred years.
I won't take credit for this since it isn't my original idea, but someone else on here pointed this out and it never clicked for me before then. American Liberals are quite fond of citing George Washington's criticisms of party politics as given in his Farewell Address.
They are very quick to pull this out of their back pocket when bemoaning the existential hostility and eternal deadlock which exists in Congress, but they never seem to realize that what Washington is essentially advocating for a one party state.
Now, I'm not the kind of nerd who looks up to George Washington as any sort of role model, but the dude is obviously respected throughout common society. I think it is good rhetorical move to pull this one out whenever you encounter Liberals dismissing political systems on the sole basis that they are one/no party states. This is a rhetorical tactic that Ho Chi Minh used famously at the outset of Vietnam's anti-colonial struggle. He quoted lines of the US Declaration of Independence verbatim to state on one hand "We're just doing the thing that you did. We're even justifying it in your own words," while on the other hand highlighting the hypocritical nature of a country which celebrates its revolutionary history while crushing revolutions all over the world.