The whole concept of being justified in fighting for territory seems to be a rather ambiguous, flexible idea that continues to evolve. There are some fundamental questions that don't really have answers in order to get a hard and fast ruling of 'yes, USA was okay in what it did" or "no, USA should give California, Texas, and rest of Southwest back to Mexico".

The types of questions we're talking about....

-Is sovereignty absolute? Did Nazis have the sovereign right to gas their own citizens? Did the Iraqis? Both of these examples are tough, because in both cases, the parties in question engaged in aggressive expansion of their own.... But to use a non-expansionist example, did the President of Yugoslavia (who granted was a puppet for Milosevic) have the sovereign right to hold onto Croatia and Bosnia at any cost? Did we have the right to force the Serbs by force of arms to stop the seccession of Bosnia? Well, we won, so yes. But if we had lost, the answer would have been no.

-How far back do territorial claims go? When does land ownership truly pass from one people to the next? Using the same arguments that the Palestinians use to justify their claim of sovereignty, we could claim that the entire area really belongs to the Canaanites (a non-semitic people) and all semites: Jews and Arabs, must vacate immediately.

-What is a 'people'? We talk about stealing the land from Native Americans and our moral obligation to give it back. But the Iriquois League and the Algonquin Nation were fighting over Vermont and New Hampshire for hundreds of years before white people ever showed up (well, there was Leif Eriksson, but you get my point).

Speaking of whom, I think they had the right idea... the Earth itself belongs to noone. It's only the resources it yields that can be claimed (horses, herds for hunting, fishing stocks, etc) and fought for.

As for the particular case in question, I have to agree with those who have said that timing was everything. Texas didn't join the USA immediately, there was a 10 year discrepancy there. As for California being part of Mexico, was it ever? I know it was part of the Spanish colonies in North America, but that's not the same thing.

In conclusion, I fall back to the old adage, history is written by the victors, and the winners are always right. It is a good thing to remember this in reviewing history, as 'right and wrong' don't really play too well. You can make good arguments for most of history's losers (the Carthaginians, the Gauls, in time the Romans themselves, the Irish, the Native Americans, the Confederacy) but in many ways, they were every bit as wrong as the winners. There's nothing enobling in losing.

Facts are value neutral. We weren't right to annex Texas and the rest of the Southwest, and we weren't wrong. We just did.