What semantic notions inverted the meaning of 'with' (from opposition to association)?

Question

[Wiktionary :] From Middle English with, from Old English wiþ ‎(“against, opposite, toward”), a shortened form of wiþer, from Proto-Germanic *wiþr- ‎(“against”), from Proto-Indo-European *wi-tero- ‎(“more apart”); from Proto-Indo-European *wi ‎(“separation”).      [...]

[Etymonline, which appears to have extracted from OED :]     [...]      Sense shifted in Middle English to denote association, combination, and union, partly by influence of Old Norse vidh, and also perhaps by Latin cum "with" (as in pugnare cum "fight with").     [...]

History confirms that enemies (who are separate from, and who fight, each other) can become friends (who must be associated and united);
1. but what semantic notions explain, conciliate and underlie the polar opposites of association and opposition? Does my facile conjecture really explain the enormity of this semantic inversion?

2. How did the Old Norse vidh and Latin cum influence the Semantic Shift of 'with'?

Answer 1

One of the many meanings of Old Norse við was "nearby, close to" and another was "in the company of" or "in the presence of". For the Norse speaker, it also meant "against, towards". See An Icelandic-English Dictionary 2nd edition (Oxford 1957).

There were many Norse speakers in England; large areas of the country were peopled (often mainly) with Scandinavians. English shows much Norse influence. Over the course of generations the language of English speakers would gradually accommodate the new meaning as the peoples had business dealings, became neighbors, and intermarried.

You might wish to read what the Middle English Dictionary has to say about "with".