This is how Wikipedia sums it, in part:
The Treaty of Guadalupe Hidalgo (Tratado de Guadalupe Hidalgo in Spanish) is the peace treaty, largely dictated by the United States (U.S.) to the interim government of a militarily occupied Mexico City, that ended the Mexican-American War (18461848). The treaty provided for the Mexican Cession of 1.36 million km² (525,000 square miles) to the United States in exchange for US$15 million (equivalent to $380 million today). From the standpoint of Mexico, the treaty included an additional 1,007,935 km² (389,166 sq mi) as Mexico had never recognized the Republic of Texas nor its annexation by the U.S., and Mexico lost 55% of its pre-war territory.
The treaty also ensured safety of existing property rights of Mexican citizens in the transferred territories. Despite assurances to the contrary, property rights of Mexican citizens were often not honored by the U.S. in accordance with modifications to and interpretations of the treaty.
These Mexicans want their land back? Well, if that’s the case, they should return Mexico to Spain.
It was their dictator whom signed the treaty. Go cry on their leaders shoulders.