Let's consider the two possibilities:
(1) Hätten die Kommunisten die Versorgung gekappt, die Sache wäre entschieden gewesen.
(2) Hätten die Kommunisten die Versorgung gekappt, wäre die Sache entschieden gewesen.
There is no doubt that version (2) is a correct sentence consisting of a main clause followed by a subordinate clause.
Personally, I would use a different style of punctuation in version (1):
(3) Hätten die Kommunisten die Versorgung gekappt... Die Sache wäre entschieden gewesen.
In my perception, version (1) and (3) evoke a little hesitation after the first part. The speaker/author makes a little pause to (let you) think about the consequences. Then the consequences are mentioned; however, not a subordinate clause is used but a main clause because that's a new thought.
Somehow this is like if the sentence was started without knowing where it should lead. Then there's a pause after which the original sentence is not completed but instead, a new main clause follows.
I think this can be compared to the following sentence structures:
(4) Ich verstehe diese Gleichung nicht, weil... ich hab mich noch nicht mit diesem Mathe-Thema beschäftigt.
(5) Ich glaube Peter ist heute krank, obwohl... vielleicht macht er auch nur blau.
In examples (4) and (5) there is a little pause between the sentences where the speaker thinks about what should follow. Since this pause acts like a hiatus, the second sentence in each example shows a main clause structure instead of a subordinate clause structure that one would expect if the sentences were properly connected without a pause.