He unfortunately did not discuss the legal basis for this re-evaluation.
However, the memo itself, written by State Department Legal Advisor Herbert J. Hansell, can be critiqued - by one of the people it quotes as an expert.
Hansell writes:
[T]he Israeli armed forces entered Gaza, the West Bank, Sinai and the Golan Heights in June 1967, in the course of an armed conflict. Those areas had not previously been part of Israel's sovereign territory nor otherwise under its administration. By reason of such entry of its armed forces, Israel established control and began to exercise authority over these territories; and under international law, Israel became a belligerent occupant of these territories.
...'In positive terms, and broadly stated, the Occupant's powers are (1) to continue orderly government, (2) to exercise control over and utilize the resources of the country so far as necessary for that purpose and to meet his own military needs. He may thus, under the latter head, apply its resources to his own military objects, claim services from the inhabitants, use, requisition, seize or destroy their property, within the limits of what is required for the army of occupation and the needs of the local population.But beyond the limits of quality, quantum and duration thus implied, the Occupant's acts will not have legal effect, although they may in fact be unchallengeable until the territory is liberated. He is not entitled to treat the country as his own territory or its inhabitants as his own subjects..., and over a wide range of public property, he can confer rights only as against himself, and within his own limited period of de facto rule.' J. Stone, Legal Controls of International Conflict, 697 (1959).'
Hansell quotes Julius Stone as the authority on what can be done under belligerent occupation. But he bases the idea that the territory is considered occupied on his own reasoning, with no references.
Three years later, Julius Stone wrote "Israel and Palestine:Assault on the Law of Nations." Within that, he writes:
It has been shown in Chapters 3 and 7 that there are solid grounds in international law for denying any sovereign title to Jordan in the West Bank, and therefore any rights as reversioner state under the law of belligerent occupation.
The grounds on which Israel might now or in the future claim to have such tide have also there been canvassed. The initial point that arises under Article 49(6) of Geneva Convention IV of 1949 is more specific. Not only does Jordan lack any legal title to the territories concerned, but the Convention itself does not by its terms apply to these territories. For, under Article 2, that Convention applies "to cases of ... occupation of the territory of a High Contracting Party," by another such Party. Insofar as the West Bank at present held by Israel does not belong to any other State, the Convention would not seem to apply to it at all. This is a technical, though rather decisive, legal point.
What about settlements?
Hansell writes:
Article 49 of the Fourth Geneva Convention relative to the Protection of Civilian Persons in Time of War, 12 August 1949, 6 UST 3516, provides, in paragraph 6:
'The Occupying Power shall not deport or transfer parts of its own civilian population into the territory it occupies'.Paragraph 6 appears to apply by its terms to any transfer by an occupying power of parts of its civilian population, whatever the objective and whether involuntary or voluntary. [Footnote: Paragraph 1 of article 49 prohibits "forcible" transfers of protected persons out of the occupied territory; paragraph 6 is not so limited.]
It seems clearly to reach such involvements of the occupying power as determining the location of the settlements, making land available and financing of settlements, as well as other kinds of assistance and participation in their creation. And the paragraph appears applicable whether or not harm is done by a particular transfer.
The language and history of the provision lead to the conclusion that transfers of a belligerent occupant's civilian population into occupied territory are broadly proscribed as beyond the scope of interim military administration.
Another view of paragraph 6 is that it is directed against mass population transfers such as occurred in World War II for political, racial or colonization ends; but there is no apparent support or reason for limiting its application to such cases.
Hansell is not a recognized legal scholar, and he is interpreting international law here without referring to any such scholars. His assumption that the background of why Article 49(6) was written has no bearing on its interpretation is almost breathtaking in its naked self-assurance.
Here is what Stone wrote about the settlements' legality, even assuming that the territory is considered occupied. Again, this is Stone's area of expertise. I am editing this fairly ruthlessly because of its length, but you can read it all here.
It is clear that in its drafting history, Article 49 as a whole was directed against the heinous practice of the Nazi regime during the Nazi occupation of Europe in World War II, of forcibly transporting populations of which it wished to rid itself, into or out of occupied territories for the purpose of "liquidating" them with minimum disturbance of its metropolitan territory, or to provide slave labor or for other inhumane purposes. The genocidal objectives, of which Article 49 was concerned to prevent future repetitions against other peoples, were in part conceived by the Nazi authorities as a means of ridding the Nazi occupant's metropolitan territory of Jews—of making it, in Nazi terms, judenrein. Such practices were, of course, prominent among the offenses tried by war crimes tribunals after World War II. ...
...Article 49, paragraph 6, uses similar language, though with significant differences, forbidding the occupying power to "deport or transfer parts of its own civilian population into the territory it occupies." Notably, paragraph 6 does not include the peremptory clause "regardless of motive," so that the spirit of its provision, as well as the letter, requires attention. Dr. Pictet's commentary acknowledges "some hesitation" and some doubts in the drafting as to its relation to the above main preoccupation of Article 49. He observes, "It is intended to prevent a practice adopted during the Second World War by certain Powers, which transferred portions of their own population to occupied territory for political and racial reasons or in order, as they claimed, to colonize those territories. Such transfers worsened the economic situation of the native population and endangered their separate existence as a race."
It is clear that historically the victims of the terrible abuses that Dr. Pictet, as well as this writer, regards as a key to interpreting paragraph 1, included many thousands who were nationals of the Nazi occupying power in Nazi metropolitan territory, and who were deported (e.g., to Poland). Many of these, for example the Jews, had shortly before the relevant time been deprived of German nationality, presumably in order to expose them more easily to arbitrary action.
If and insofar, therefore, as Israel's position in Judea and Samaria (the West Bank) is merely that of an occupying power, Article 49 would forbid "deportation" or "transfer" of its own population onto the West Bank whenever this action has the consequence of serving as a means of either (1) impairment of the economic situation or racial integrity of the native population of the occupied territory; or (2) inhuman treatment of its own population.
Impairment of Racial Integrity of the Native Population of the Occupied Territory. ...Despite vociferous political warfare pronouncements on both sides, it seems clear, therefore, that no serious dilution (much less extinction) of the "separate racial existence" of the native population has either taken place or is in prospect. Nor do well-known facts of dramatic improvement in the "economic situation" of the inhabitants since 1967 permit any suggestion that that situation has been worsened or impaired.
Insofar, moreover, as these or future settlements are merely directed to the requirements of military security in the occupied territory they do not violate either the spirit or the letter of this aspect of Article 49. And they also conform, as the preceding discourse has shown, to the general requirements of customary international law, embracing the relevant provisions of the Fourth Hague Convention of 1907, and its annexed regulations.
Inhuman Treatment of the Occupant State's Own Population. The second aim of the prohibition in Article 49(6) was, as has been seen, to protect the inhabitants of the occupant's own metropolitan territory from genocidal and other inhuman acts of the occupant's government. That this was part of, if not the main intention of Article 49(6) seems clear from the use of the term "deport," which clearly refers to a coerced movement of its population. The addition of the term "or transfer" does not alter this import. ...the word "transfer" in itself implies that the movement is not voluntary on the part of the persons concerned, but a magisterial act of the state concerned.
As contrasted with this main evil at which Article 49 was aimed, the diversion of the meaning of paragraph 6 to justify prohibition of the voluntary settlement of Jews in Judea and Samaria (the West Bank) carries an irony bordering on the absurd. Ignoring the overall purpose of Article 49, which would inter alia protect the population of the state of Israel from being removed against their will into the occupied territory, it is now sought to be interpreted so as to impose on the Israel government a duty to prevent any Jewish individual from voluntarily taking up residence in that area. For not even the most blinkered adversary of Israel could suggest that the individual Jews are being in some way forced to settle in Judea and Samaria (the West Bank)! The issue is rather whether the government of Israel has any obligation under international law to use force to prevent the voluntary (often the fanatically voluntary) movement of these individuals.
On that issue, the terms of Article 49(6), however they are interpreted, are submitted to be totally irrelevant. To render them relevant, we would have to say that the effect of Article 49(6) is to impose an obligation on the state of Israel to ensure (by force if necessary) that these areas, despite their millennial association with Jewish life, shall be forever judenrein. Irony would thus be pushed to the absurdity of claiming that Article 49(6), designed to prevent repetition of Nazi-type genocidal policies of rendering Nazi metropolitan territories judenrein, has now come to mean that Judea and Samaria (the West Bank) must be made judenrein and must be so maintained, if necessary by the use of force by the government of Israel against its own inhabitants.
Hansell's short stint at the State Department was over before Stone's book, but the breadth of the legal analysis by Stone blows away anything done by Hansell.