could have stemmed [closed]

What is the exact meaning of the phrase "could have stemmed from" in the following passage? The words in bold are intended:

The role of the Tudeh and its support of the Soviet Union's demand for an oil concession in the north or Iran were clear evidence of their closeness, especially when members of the party took to the streets in large numbers in favor of the Soviet bid. On balance, it could be argued that their support could have stemmed from a nationalist need to counter-balance Britain's predominance in Iran.

The ambiguous point is that their support was really stemmed from a nationalist need or not. Could have pp means that something was possible in the past, or you had the ability to do something in the past, but that you didn't do it. But a am not sure about this function in the above passage. I should say Tudeh was an Iranian communist party famous for supporting Russia.

This usage is defined at https://www.merriam-webster.com/dictionary/stem as

intransitive verb

: to occur or develop as a consequence : have or trace an origin

her success stems from hard work

You could rephrase the sentence as

On balance, it could be argued that their support could have been because of a nationalist need to counter-balance Britain's predominance in Iran.

Close alternatives would be "might have arisen from" or "could have been caused by".

"Stemmed" is being used as a plant metaphor (like "grown" or "blossomed"). It's a verb, but a metaphoric one, so the passive "was really stemmed" doesn't really work - though the active "their support really stemmed" would.

It's not so much ambiguous as intentionally vague - the writer can't demonstrate for certain whether the support was or wasn't caused by "a nationalist need", so has left that up to the reader to infer.

• ... it could be argued that their support could have stemmed from ...

That's two could's, one modifying the other; since could indicates possibility, but not probability, each proposition has less than .5 likelihood. Since one composes with the other, the product rule applies, and the resultant likelihood is quite low. Two propositions of likelihood .4, for instance, have a joint likelihood of .16. In effect, phrasing things this way is a polite brushoff.

Could have pp means that something was possible in the past, or you had the ability to do something in the past

No, in this case it is expressing uncertainty, as could is a strongly inflected subjunctive form of can. The subjunctive has significantly eroded in English (hence I think this question is relevant for ELU, even if your broken English would almost imply it was a better fit for englishlanguagelearning.SE). This is called potentialis, which is distinct from irrealis or the quite factual negative expressions (i.e. cannot) in past tense (could not). Adding have to form past tense makes it clear that could itself is not marking the past, as that would be redundant.