The Communist Manifesto is really a manifesto of state-capitalism. The "conquest of political power by the proletariat" really means the conquest of state power by a political party that claims to represent the proletariat. Upon taking power that party then ends up waging war against and subjectating the proletariat.
Establishing a state controlled by the proletariat, as advocated by the
Manifesto, is not possible and attempts to do so leads to the formation of a new group of exploiters to replace the old ones. The state is an organisation with a monopoly (or near-monopoly) on the legitimate use of violence. It is a centralized rule-making body that stands "above" society and uses various armed bodies of people and coercive institutions (courts, prisons, etc.) to force people to obey it. It is an organ of class rule which cannot be used to abolish classes. How are the workers supposed to maintain control of an organisation standing "above" society with it's own specialized armed forces and maintaining a monopoly on the legitimate use of violence? It's not possible - the state is the one with a monopoly on violence and could use that monopoly to ignore what the proletariat want and order them around - effectively forming a new ruling class over the proletariat. The myth of the "good state" is a popular one among leftists, but in reality every state has been founded on the blood of the poor (even "socialist" ones.
Liberalism and Authoritarian Socialism both share a common theme in that they establish systems of minority rule and claim that this system of minority rule is actually majority rule; that the rulers aren't really the rulers. With Liberalism they claim that under their state "the people" rule but the wealthy (and corrupt politicians) actually rule; with Marxist-Leninism they claim that under their state "the proletariat" rule but actually the party (or, more acurrately, the leaders of the party) rules.
In a centrally planned economy, instead of decisions being made by the producers themselves decisions are made by a small group of centralized planners in Moscow (or Washington or London or some other capitol). The workers are disempowered, deprived of control of their own lives, and forced to submit to these planners. Material conditions have a huge impact on a persons consciousness, behavior, and material interests. Individuals are shaped by the institutions they are a part of, the position they occupy in those institutions and the social relationships they have with others. Since they are in different conditions then the workers these buerecrats will tend to end up with different consciousness and material interests. There's no reason to expect them to act in the workers interests, and since they have different material interests then the workers will come into conflict with the workers (a conflict called class struggle). This happens even if your buerecrats are elected workers as, once elected, they are no longer workers but buerecrats. Thus the actual rulers are not workers but buerecrats who end up constituting a new ruling class that exploits the proletariat just as the previous ruling class did. It doesn't matter whether this is applied in a single isolated country, a third of the globe or the entire world - this is inherent in the nature of a centrally planned economy.
Joe