How do you see the relationship between war and legality?
The US acquired territory from Mexico as part of the treaty of Guadalupe Hidalgo, intended to end the Mexican-American war. Mexico lost 55% of its territory, but did receive $15M as some sort of compensation. Is that legal and if so in what sense, given that it was driven by a desire to end a war?
Much more recently, how about territory acquired by Russia in the vicinity of Ukraine. Suppose that the Crimeans truly did unanimously agree to this, would that make it legal? Is it legal if 50% of the residents agree? What about those with economic interests there who are not residents?
In modern times, the rule has been that a nation's territory can only be permanently sold with the consent of its government, and that government constitutes the sole authority whose agreement is needed.
Sub-areas within a nation may secede and form their own nations, but recognition of the new nations by peer nations is required for legitimacy.
So from that perspective: Mexico, agreed to by the government and done. Crimea, not consented by the Ukrainian government, and an open question as to peer recognition of independence (or now incorporation into Russia).
The grey areas (e.g. ceding land under duress) are why land has historically been the chief casus belli.
The US acquired territory from Mexico as part of the treaty of Guadalupe Hidalgo, intended to end the Mexican-American war. Mexico lost 55% of its territory, but did receive $15M as some sort of compensation. Is that legal and if so in what sense, given that it was driven by a desire to end a war?
Much more recently, how about territory acquired by Russia in the vicinity of Ukraine. Suppose that the Crimeans truly did unanimously agree to this, would that make it legal? Is it legal if 50% of the residents agree? What about those with economic interests there who are not residents?