Earlier quoted context omitted.
There are activists who argue that all of the southwestern US should be returned to Mexico. And indeed, the idea generally isn't that unpopular in Mexico. For what that's worth.
Californian here. Why would that be “offensive”?
http://www.paulgraham.com/say.html
"The statements that make people mad are the ones they worry might be believed."
At present, the idea that California might be returned to Mexico or that all non-native-Americans might be kicked out of America is sufficiently ridiculous that nobody will take you seriously. Thus, there's no reason to get offended. You can say whatever you want, you're just a nutcase.
That China gets mad whenever you suggest that Tibet become its own country or when you mention Tiananmen Square indicates that it worries its citizens might actually take you seriously...