Correct and the legal argument these subversives make is that the the Treaty of Guadeloupe-Hidalgo, which ended the Mexican War in 1845, was illegally forced upon Mexico.
Therefore these lands belong to Mexico.
I love when they bring out the "We didn't cross the border, the border crossed us." bull excrement.
I like to point out at the time that occurred, only the people living in what is now United States territory had the border "cross them", and they are long dead. Those from south of there have no claim to that argument.
When you lose a war, the victor gets to impose the conditions.
'Illegally' dictating terms is a silly assertion when the conquering army is occupying your capital city..