Present tense / past tense

"Stalin's initiatives to nurture closer relationships with left-wing but non-Communist movements in Western Europe do not in themselves prove the supposition of an aggressive Soviet Union on the offensive. They are best regarded as part of Moscow's larger attempt to create a web of expedient working relationships with forces of the Left."

Say I am making these assertions against some scholars who disagree with me. Can an equally well-supported case be made for the use of either "do not/are" or "did not/were" in the above passage? Or is only one of the two pairs possible?

A related example:

"Of the seven strategic errors Napoleon committed when invading Russia, letting his line of supply overextend is the gravest."

Same question: it seems that most people would use 'was' here, but then is 'is' really unacceptable? Or is the choice of 'is' still acceptable if odd?

Thank you.