Mexico Falling into the Protective Arms of Loan Sharks
While Lincoln’s diplomats were assuring Mexico that they would protect its independence, their armies were fighting against the independence of the American South.  United States Minister to Mexico, Thomas Corwin, was able to purchase the cooperation of Mexico with loans despite their very low opinion of Secretary of State William Seward, “whose expansionist proclivities included not only Mexico and Central America, but also Canada in his wild dreams of “Greater America.”
Bernhard Thuersam, Chairman
North Carolina War Between the States Sesquicentennial Commission
"The Official Website of the North Carolina WBTS Sesquicentennial"
Mexico Falling Into the Protective Arms of Loan Sharks:
“Having convinced Mexico of the unfriendly designs of the Confederacy and to that extent checkmated Confederate diplomacy, Corwin and Seward reaped their first material fruits by obtaining the privilege of marching [Northern] troops across Sonora from California into Arizona. So well had the diplomacy and propaganda of the United States worked that when the question of allowing the passage of troops was considered in the Mexican Congress, this passed by unanimous vote. 
Perhaps the most effective work done by Corwin in heading off Southern diplomacy and intrigue in Mexico was the effort he made to induce the United States to lend Mexico first nine, and later eleven, million dollars.  Corwin determined to risk eleven million dollars partly for the purpose of defeating Confederate designs in Mexico and preventing Confederate recognition, and partly to defeat European intervention in Mexico.
Corwin suggested that this loan be made to [Benito] Juarez to put him in a proper state of mind as well as of defense before [Confederate diplomat John T.] Pickett arrived in Mexico City.  While Pickett was holding his interviews, Corwin urged the Mexican loan treaty, to make sure that no favors should be shown the Confederacy.  His first recommendation to [Secretary of State William] Seward was in his second dispatch of June 29….[urging] a loan of several millions be made to put Mexico in a state of defense against her Confederate and European enemies and to pay off the interest on the debts owed Spain, France, and England.
“Mexico,” [Corwin] said, “would be willing to pledge all her public lands and mineral rights in Lower California, Chihuahua, Sonora, and Sinaloa, as well as her national faith for the payment of this guarantee.  This would probably end in the cession of the sovereignty to us.  It would be certain to end if the money were not promptly paid as agreed on.”
He would not advise such a step if Mexico were able to save herself from the despoiling hands of the Confederates and Europeans. But that seemed impossible. Besides, it would be only the good fortune of Mexico if that unhappy nation fell into the protecting arms of the United States, for “the United States are the only safe guardians of the independence and true civilization of this continent. It is their mission and they should fulfill it.”
Besides, the annexation of these Mexican states would make up for the loss of the Southern States. “England and Spain are now in possession of the best of the West Indies….and Mexico, a colony of England with British power on the North of our possessions, would leave on the map of this continent a very unimportant part for the United States – especially should the present unnatural rebellion end in the final severance from us of eight or nine or all of the slave States.”
Mexico must not be allowed to go to England; rather than that, the United States must have her, and the best way to prevent England or the Confederacy from getting her and to obtain a good portion for the United States was the proposed treaty with Corwin urged upon Seward.”
(King Cotton Diplomacy, Frank Lawrence Owsley, University of Chicago Press, 1931, pp. 106-109)