how is it associated with the mesilla valley?
While determining the boundaries of the U.S. and Mexico, there was a disputed territory involved a few thousand square miles and about 3,000 residents; more significantly, which included the Mesilla Valley.
Gadsden Purchase
 The disputed territory involved a few thousand square miles and about 3,000 residents; more significantly, it included the Mesilla Valley. 
who negotiated the purchase of the area south of gila river UNANSWERABLE what is this purchase all about? It is a region of present-day southern Arizona and southwestern New Mexico that the United States acquired from Mexico by the Treaty of Mesilla.
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Gadsden Purchase [SEP] Mesilla Valley
Southerners opposed this alternative because of its implication for the railroad, but President Fillmore supported it. Southerners in Congress prevented any action on the approval of this separate border treaty and eliminated further funding to survey the disputed borderland. Robert B. Campbell, a pro-railroad politician from Alabama, later replaced Bartlett. Mexico asserted that the commissioners' determinations were valid and prepared to send in troops to enforce the unratified agreement.