Kontrovich started off by saying what international law is not. It is not UN General Assembly resolutions. It is not advisory opinions from the ICJ (which, he pointed out, was answering a loaded question that assumed illegality when it gave its opinion on the security fence.)
The first legally important act after the fall of the Ottoman Empire that is relevant to Israel's borders is the League of Nations Mandate for Palestine, which noted the "historical connection of the Jewish people with Palestine and to the grounds for reconstituting their national home in that country." After the British partitioned Western Palestine from Trans-Jordan, the implication is that all of the remaining Palestine would be the area of the Jewish nation.
If the Arabs had accepted the 1947 Partition Plan, then the further partition of Palestine into an Arab and Jewish state would have legal weight. But since they didn't, the Jewish claim on all of Palestine remained in force.
The 1949 Armistice Lines (mistakenly called the "1967 borders") are emphatically not national boundaries. They are explicitly stated in the armistice agreements as "not to be construed in any sense as a political or territorial boundary, and is delineated without prejudice to rights, claims and positions of either Party to the Armistice as regards ultimate settlement of the Palestine question" (from the Egyptian armistice,the Jordanian one says "without prejudice to future territorial settlements or boundary lines or to claims of either Party relating thereto.") Their position (generally) had no demographic, political or geographic significance; they were simply where the opposing armies ended up at the last truce, with some minor adjustments. From the perspective of international law, they are not borders.
Jordan's sovereign claims to the West Bank were not recognized by the international community.
The next important legal document is UN Security Resolution 242 at the end of the 1967 war. (While it is a Chapter 6 resolution, Kontotovich noted that it was referred to in some Chapter 7 resolutions, meaning it might have the strength of the stronger Chapter 7 resolutions itself with respect to international law.) He discussed the famous missing "the" from the phrase "Withdrawal of Israeli armed forces from territories occupied in the recent conflict" and noted that this was done deliberately to make the resolution purposefully ambiguous as to whether Israel must withdraw from all the territories. He noted that in the end, when Israel relinquished the Sinai and later Gaza, Israel had withdrawn from some 99% of the territories, so it cannot be accused of violating the spirit of the resolution.
He also noted that the legality of Israel's annexation of the Golan Heights is a completely different topic from a legal perspective, and he did not get into it.
Kontorovich said that there is a big question in international law about whether one can legally acquire territory via conquest in a defensive war. He looked up five sources written before 1967 on the question; 2 said yes, 2 said no and one didn't think about it. In the case of the 1967 war, as with many things about Israel, the legal issues are completely unique and anyone saying that international law says something definitively on something that never happened before is generally not to be trusted. (I asked him whether the preamble of 242 meant that the UN considered the war not to be defensive; he answered that besides the fact that preambles are not part of the law, it would not make sense to interpret it that way because in that case Jordan also couldn't lay claim on it. He concluded that it was placed there in order to encourage the parties to come up with a negotiated border, as 242 states, and not a border created by conquest.)
The next legally important event for determining Israel's borders was the 1993 Oslo Accords. This is where Israel is relinquishing part of its occupied land (he noted that from Israel's perspective the land is occupied since it was not annexed, although it is legally occupied) to give to an ultimate Palestinian Arab entity.
Here is where he said something new.
In Professor Kontorovich's opinion, at some time after Oslo, Palestine became a state under international law.
The definition of a state is given by the Montevideo Convention. I had argued, and so had others, that "Palestine" does not constitute a state under its definition:
The state as a person of international law should possess the following qualifications: (a) a permanent population; (b) a defined territory; (c) government; and (d) capacity to enter into relations with the other states.
Kontorovich didn't get into the issue of Gaza, which may seem to be a problem since it has a different government, but he argues that since Area A is unquestionably part of what is claimed to be Palestine, that West Bank entity is undoubtedly a state. Having defined territory is not the same as having defined borders, and "Palestine" has st least some territory that it can call its own.
Therefore, the professor says, the entire issue nowadays between Israel and the Palestinian Arab state is not an issue of occupation or legality - it is simply a border dispute that must be resolved the way all border disputes are resolved (or not.)
One other point, not dwelled on in his slides, is that the Fourth Geneva Conventions article 49 on transferring people to occupied territory does not apply because in Israel's case the people moved there voluntarily, and Geneva implies government organized mass transfers.
It was a very interesting and thought provoking talk, and I spoke to him afterwards; he's a really nice guy. He's doing a fellowship in Princeton now but he is based out of Northwestern University.
If you want to hear him talk, here are some upcoming presentations he is making:
Cherry Hill, NJ on Feb. 12, One State, Two State, Three State, Four: The PA Bid for UN Recognition, sponsored by the RJC Southern New Jersey Chapter:
Princeton, NJ Feb. 12: International Lawfare, BDS. and the Delegitimization of Israel, sponsored by Advocates for Israel.
University of Florida Law School, Feb 15 at 12:15, Disputing Occupation: Israel's Borders in International Law
East Windsor, NJ March 28th: Say It Enough, it Still Isn't True: Illegal Occupation, Settlement, and Apartheid, sponsored (and hosted) by Beth El synagogue and Speak Up for Israel.