The major legal reason given by critics of Israel today against the annexation of the Golan Heights is a combination of what the UN Charter says with an unwritten but widely assumed corollary.
The UN Charter says in Article 2, paragraphs 3 and 4:
All Members shall settle their international disputes by peaceful means in such a manner that international peace and security, and justice, are not endangered.The corollary, which as far as I can tell wasn't made explicit until the preamble of Resolution 242 in the wake of the Six Day War, is that acquisition of territory in war is invalid:
All Members shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any state, or in any other manner inconsistent with the Purposes of the United Nations.
"Emphasizing the inadmissibility of the acquisition of territory by war and the need to work for a just and lasting peace in which every State in the area can live in security,"This was finally made prescriptive in UNGA 2625 of 1970: "The territory of a State shall not be the object of acquisition by another State resulting from the threat or use of force. No territorial acquisition faulting from the threat or use of force shall be recognized as legal."
Does this apply in wars of self-defense as well? Today, most legal scholars argue that it does. Before 1967, however, their views were split.
It seems logical that the corollary of the illegality of gaining territory through force is that the aggressor should not be rewarded for his illegal aggression. If the party gained land in self-defense, and self-defense is legal under the UN Charter, then the spirit of the rule is maintained - the aggressor is not rewarded for his aggression. Otherwise the aggressive party can keep trying to destroy his enemy over and over again with no repercussions.
Possibly, the most relevant document from before 1967 that deals with this issue is the 1949 Draft Declaration on Rights and Duties of States which was an early attempt by the UN to codify these sorts of issues written by its International Law Commission. I'm not certain of its legal status but it was based on the best UN international law expert opinion of the time.
It says in articles 9, 11 and 12:
9. Every State has the duty to refrain from resorting to war as an instrument of national policy, and to refrain from the threat or use of force against the territorial integrity or political independence of another State, or in any other manner inconsistent with international law and order.It says explicitly that the only time that acquisition of territory should be considered illegal is when it is acquired during an illegal war, in violation of Article 9. A war of self defense is legal (in fact the only kind that is legal,) as Article 12 says.
11. Every State has the duty to refrain from recognizing any territorial acquisition by another State acting in violation of article 9.
12. Every State has the right of individual or collective self-defence against armed attack.
The idea that acquisition of territory in a defensive war is illegal seems to have only gained traction after 1967. Interesting how international law always seems to morph against Israel and only Israel! But it is important to recognize that the evolution of international law does not work retroactively - if acquisition of territory in a defensive war was legal before 1967, then Israel's control over the Golan remains legal today, even if today it is accepted that defensive acquisition is not acceptable.
As legal scholar Eugene Kontorovich notes, there were many other cases of legal acquisition of territory by war between 1949 and 1967:
The views of the U.N’s International Law Commission and most scholars in finding defensive conquest as lawful under the U.N. Charter should not be surprising given that it simply reflected broad state practice under the Charter. In the years immediately following the adoption of the Charter, many of the victorious Allies took territory of the defeated nations. All these annexations have been recognized, without controversy by the U.S. and international community. To mention only a few of these instances, Holland unilaterally annexed parts of Germany in 1949; Greece and Yugoslavia took parts of Italy; the U.S.S.R and Poland annexed large parts of Germany. The ILC in its deliberations specifically addressed the legal basis for these annexations: because the underlying use of force was lawful (defensive), the acquisition of territory can be permitted.No one disputes any of these. Only when Israel is involved does international law suddenly change, always to Israel's detriment.
Nor did this practice stop with the immediate aftermath of WWII in the 1940s. At the close of the Korean War in 1953, the Republic of Korea controlled and claimed sovereignty of portions of territory north of the pre-war boundary at the 38th parallel. Nonetheless, the U.S. and the international community has not seen any obstacle to recognizing Seoul’s sovereignty over this territory.