The west bank annexation happened in response to Jordanian aggression (who were joining with Egypt, with whom Jordan had a mutual defense pact, in a war I would say was ambiguously instigated by Israel/Egypt).

Israel holds the territory with its army, and claims that neither Jordan (nor any country) owned it beforehand, which is a better-founded argument than some people may expect - the west bank was originally, by england, designated as a palestinian - not jordanian - state, arab leadership rejected it (to also reject an israeli state), england withdrew from it, and jordan occupied it, with very very limited international recognition.

It's a pretty crazy history, I hope I got it right, please fact-check me.