(Israeli settlement of Migron*. Even the Israeli Supreme Court agrees this one is illegal.)
I've been asked a couple of times about whether or not the Israeli settlements built on territory occupied in the 1967 Six Day War are legal, and if not, why not - given that the Israeli Government distinguishes between legal and illegal settlements in the West Bank.
The Israeli Ministry of Foreign Affairs marshals an extensive argument on their website explaining why Article 49(6) of the Fourth Geneva Convention 1949 which forbids an occupying power from "deport[ing] or transfer[ing] parts of its own civilian population into the territory it occupies" doesn't apply in the West Bank, the Golan Heights (and formerly, Gaza). The Israeli MFA's claims that:
"The provisions of the Geneva Convention regarding forced population transfer to occupied sovereign territory cannot be viewed as prohibiting the voluntary return of individuals to the towns and villages from which they, or their ancestors, had been ousted."
This is arguably true but irrelevant. It is interesting that even the Israeli MFA makes the case that ancestors - presumably back to biblical times - present a legal basis for the appropriation of land and the construction of settlements. In legal terms this is a nonsense, and in practical terms it is hopeless - it would appear to give Italians legitimate title to most of the Mediterranean world, for instance.
The Israeli MFA goes on:
"Nor does [Article 49] prohibit the movement of individuals to land which was not under the legitimate sovereignty of any state and which is not subject to private ownership. In this regard, Israeli settlements have been established only after an exhaustive investigation process, under the supervision of the Supreme Court of Israel, designed to ensure that no communities are established on private Arab land."
There are at least two problems with these assertions. First, the Israeli MFA in asserting that the territories it occupied in 1967 were "not under the legitimate sovereignty of any state" implies that it was terra nulius. Simply put, this isn't true: in 1967 the West Bank and East Jerusalem were either under the sovereignty of Jordan, or it was illegally occupied by Jordan with rights reverting to the previous legitimate sovereign. (I assume that in this case the previous legitimate sovereign was the UN as the League of Nations mandate was handed back by Britain, though I'd have to do some more work on this.) In any event, the West Bank and the other other occupied territories were not terra nulius - because if they had been, then in 1967 the UNSC would not have passed Resolution 242 calling for the withdrawal of Israeli forces from "territories occupied in the recent conflict".
Second, the record is clear that Israel does appropriate Arab land, and has used it for building settlements.
In any event, the UN Security Council made clear in 1967 that the Geneva Conventions applied to the occupied territories in Resolution 237, and in 1979 that the settlements are illegal in Resolution 446, OP1 of which reads that the Security Council
"Determines that the policy and practices of Israel in establishing settlements in the Palestinian and other Arab territories occupied since 1967 have no legal validity and constitute a serious obstruction to achieving a comprehensive, just and lasting peace in the Middle East."
Indeed - and the situation has only deteriorated since then.
*Migron is significant as in a major victory for Peace Now, the Israeli Supreme Court ruled in August 2011 that Migron was illegal and demanded that the Israeli government remove it by April 2012, which is the first time this has happened in the West Bank.