Origins
Few English words have undergone as dramatic a semantic reversal as 'with.' In Modern English, it expresses accompaniment, possession, and instrumentality — the idea of togetherness. Yet in Old English, 'wið' meant 'against, opposite, in opposition to' — the idea of separateness and conflict. The word has reversed its meaning over a thousand years.
It descends from Old English 'wið' (against, opposite, toward, in exchange for), from Proto-Germanic *wiþra (against, opposite), from PIE *wi-tero-, a comparative form of *wi- (apart, away, in two). The PIE root *wi- expressed separation and division, and its comparative *wi-tero- meant 'more apart, further away' — which in the Germanic languages became 'against, facing in opposition.'
The semantic shift from 'against' to 'with' occurred during the Middle English period, roughly between 1100 and 1300 CE, and was likely influenced by contact with Old Norse 'við,' which could mean both 'against' and 'with, by, at.' Scandinavian settlement in England during the Viking Age brought Old Norse 'við' into daily contact with its English cognate 'wið,' and the Norse comitative sense gradually transferred to the English word. The older English preposition for accompaniment, 'mid' (cognate with German 'mit'), was simultaneously declining in use and was eventually displaced entirely by 'with.'
Latin Roots
The logic of the semantic shift is not as paradoxical as it first appears. To fight against someone, you must be with them — face to face, in close proximity. Opposition implies co-presence. The same semantic bridge exists in other languages: French 'avec' (with) derives from Latin 'apud hoc' (at this, near this), and the Latin preposition 'cum' (with) developed from a sense of physical proximity rather than emotional solidarity.
The old adversative meaning of 'with' is perfectly preserved in several English compounds. 'Withstand' means to stand against. 'Withdraw' means to draw away (the 'with' here means 'away, back' — i.e., in opposition to the current position). 'Withhold' means to hold back. 'Wither' may derive from a related sense of 'against the weather' — to be worn by exposure. In each case, 'with-' retains its original meaning of opposition, resistance, or separation.
German 'wider' (against) and 'wieder' (again) are both cognates of 'with,' preserving the adversative sense. The compound 'Widerstand' (resistance) mirrors English 'withstand' exactly. Swedish 'vid' (at, by, near) and Icelandic 'við' (against, with) show the full range of meanings this word has carried across the Germanic languages.