What happened
Israel built the barrier from 2002 in response to the Second Intifada's suicide bombing campaign, and Israeli governments across the political spectrum credit it with substantially reducing successful attacks in the years after its construction — a security rationale the ICJ's opinion did not dispute or evaluate on its merits, since the case concerned the barrier's routing rather than the underlying right to build a security barrier at all. The core legal finding was that roughly 85% of the barrier's planned route ran inside the West Bank rather than along the pre-1967 Green Line, in some places looping deep into Palestinian territory specifically to place Israeli settlements on the barrier's western, Israeli-controlled side.
The ICJ's advisory opinion was non-binding (advisory opinions are not enforceable court judgments) and Israel did not participate in the merits proceedings, arguing the UN General Assembly lacked standing to refer the question and that the Court could not adequately assess the security considerations involved without hearing Israel's own evidence. Fourteen of the fifteen judges nonetheless found the barrier's route, insofar as it deviated from the Green Line, violated Palestinian rights to freedom of movement and self-determination and should be dismantled with compensation paid — a finding Israel has never complied with, and construction continued substantially along its originally planned route in the years following the ruling.