This isn't the case. Marx differs from other ideologies in that it was Karl Marx himself that wrote The Communist Manifesto.

I can't do the same for most other ideologies; there's no Conservative Manifesto as it were.

With that in mind we can take Marx's book and compare it against history in these countries. If you do this then you will come to the conclusion that these regimes were not Marxist and in fact not even close to socialism.