On Acts of War, Preemptive Strikes, Defense Pacts, and Offensive vs Defensive Wars

Question

Imagine Countries G, O, and Y all border Country S.

Now imagine the following chain of events:

1. Years prior, Country S had declared that closing a certain maritime access corridor to that country would be considered a declaration of war.
2. Countries G and Y sign a mutual defense agreement.
3. In response to a (false) report that Country S was amassing military forces along Country Y's border, Country G begins amassing their military on the G/S border, and takes up positions overlooking the aforementioned maritime access corridor.
4. Country S reiterates their declaration that closure of said corridor would be considered an act of war.
5. Country G closes said corridor to Country S's shipping.
6. Countries G and O sign a defense pact.
7. Country O begins their own military build-up.
8. Fearing their ability to withstand a war on two fronts, Country S launches a preemptive strike against Country G.
9. Country O begins attacking Country S.
10. Country S defeats Country O's attacks, and captures territory from them.

Now, the war as a whole is, strictly-speaking, an offensive war initiated by Country S against Country G. However, since Country O then initiated hostilities of their own against Country S, to whom does the territory that Country O lost to Country S legitimately belong according to international law?

Answer 1

The Kellogg–Briand Pact (1928) effectively criminalizes war and conquest. So Country S would be in violation of this act.

However in matters of international law the willingness of the country to subject itself to that judgement and the willingness of the most powerful nations to enforce the prohibition is what really matters. So even though Country S is probably on the wrong side of the Pact, if the most powerful countries perfer not to follow the pact, and allow Country S to annex the land taken from Country O then it does not really matter who is right in the law.

Answer 2

There's no such thing, in reality, as "legitimately belong according to international law". Only "what great powers consider OK" and "what you have the strength to keep".

Thus, territories captured by USSR as a result of WWII/GPW (Kaliningrad/Kunisberg, and 4 Japanese islands, and parts of Poland and Romania) are all "legitimately" part of USSR. Recovered Territories - Brandenburg, Pomerania and Silesia are legitimately part of Poland (there was also territory USSR acquired Eastern polish territory in exchange).

Dutch annexation is an interesting example: they had extensive annexation plans (As per Wiki, in its most ambitious form, this plan included the cities of Cologne, Aachen, Münster and Osnabrück, and would have enlarged the country's European area by 30 to 50 percent). But that plan was vetoed by USA (note: not international laws or some international community). However, 69 km^2 of territory WAS annexed by the Dutch, and only returned after a huge reparation payment was done by Germany (in other words, it would have been kept if Germany didn't pay up, with no objections from international community or law).