President Lincoln never said it, but that was what he was thinking. One objective of Lincoln’s foreign policy was to deter European countries from recognizing the confederacy as a nation. This objective undoubtedly dissuaded Lincoln from enforcing the Monroe Doctrine when the French invaded Mexico in 1861; by June of 1863 the French occupied Mexico City and the Mexican President was on the run. A natural U.S. response would have been to support our republican neighbor’s effort to repel the European invaders. Instead, Lincoln chose to leave Mexico to its own means, so as not to push France into the rebel camp.
The Mexicans pleaded for arms, if not via outright aid, then through purchases. Mexican Ambassador Matías Romero’s purpose in Washington City was to convince the U.S. government to support the elected president of Mexico, Benito Juárez, and oppose Napoleon III’s military intervention in Mexico. Romero befriended the Lincolns—both Abraham and Mary—but was unable to convince President Lincoln to change his course. Romero shifted his attention to the U.S. Congress.
With Romero’s blessing, U.S. Senator James McDougall (Democrat, California) presented several resolutions condemning the French invasion and demanding U.S. aid to the Republic of Mexico. In the House, Congressmen John Kasson (Republican, Iowa) and Henry Davis (Republican, Maryland) objected to Lincoln’s cautious response to the French occupiers. Although these efforts garnered attention, in the end, they manifest neither in legislation nor military aid to Mexico.
An example of the passion that some Americans held on this issue is apparent in correspondence from Surveyor-General of California and Nevada (appointed by Lincoln in 1861) E. F. Beale, e.g., “do this act [stop the exportation of arms to Mexico], go and bury your dead infant in the same grave in which you will have buried Mexican independence. . . I implore you in the name of your party, of your own hopes and your country, in the name of republican institutions in the name of the great commons of America, who will assuredly impeach you to all posterity if you betray freedom in their name, and finally in the sacred catholic name of liberty itself to give these arms to Mexico.”
Similar to Romero’s campaign in Washington, was that of Plácido Vega in San Francisco. President Juárez dispatched General Vega to San Francisco to acquire weapons and funds for the Mexican Army. Vega sought to obtain arms, with mixed results, both openly and clandestinely. Vega openly declared his mission as “procuring the means necessary for repelling the usurpation of Maximilian in Mexico.” On one occasion, the Commander of the U.S. Department of the Pacific, Major General Irvin McDowell, advised Vega, “the seizure after you had attempted to secretly carry off the arms, were acts done in conformity to the most explicit and peremptory order of His Excellency the President himself.” Similarly, the U.S. Customs Collector in San Francisco, “informed General Vega of the existence of Executive orders forbidding the export, and stated the same would be enforced by me.”
What order were they referring to? On November 20, 1862, President Lincoln signed an Executive Order “Prohibiting the Export of Arms of Munitions of War.”
Washington City, November 20, 1862
Ordered, That no arms ammunition, or munitions of war be cleared or allowed to be exported from the United States until further order: that any clearances for arms, ammunition, or munitions of war issued heretofore by the Treasury Department be vacated if the articles have not passed without the United States, and the articles stopped; that the Secretary of War hold possession of the arms, etc., recently seized by his order at Rouses Point, bound for Canada.
The purpose of this order was to sustain the fragile neutrality of the European states, especially France and England. Although sympathetic to President Juárez and the Republic of Mexico, Lincoln and the Union could not afford to compel France to enter the American Civil War on the side of the “insurgents.”
On May 3, 1865, President Johnson rescinded the 1862 executive order prohibiting the exportation of arms, as well as the 1863 extension to livestock. The U.S. began providing overt and covert aid to Juárez. In 1866 and 1867, Napoleon withdrew French troops from Mexico. The Mexican “emperor” Maximilian was executed on May 19, 1867.
P.S. This research was completed while writing my second novel (to be published in 2022)
 Jason H. Silverman, “A Most Unlikely Friendship: Abraham Lincoln and Matias Romero,” January 26, 2017 (accessed May 10, 2022, https://www.lincolncottage.org/a-most-unlikely-friendship-abraham-lincoln-and-matias-romero).
 Marvin Goldwert, “Matías Romero and Congressional Opposition to Seward's Policy toward the French Intervention in Mexico,” The Americas, July, 1965, vol. 22, no. 1, pp 22-40. Senator McDougall’s resolutions were presented on January 19, 1863; January 11, 1864; March 22, 1864; and July 2, 1864.
 E.F. Beale letter to Charles James, July 16, 1864, (accessed May 10, 2022: https://ehistory.osu.edu/books/official-records/106/1090).
 Vega letter to Major General Irvin McDowell (Commander of the U.S. Department of the Pacific), November 2, 1864, (accessed May 10, 2022: https://ehistory.osu.edu/books/official-records/106/1038).
 McDowell letter to Vega, November 26, 1864, (accessed May 10, 2022: https://ehistory.osu.edu/books/official-records/106/1073).
 Charles James, Customs Collector, letter to McDowell, December 14, 1864 (accessed May 10, 2022: https://ehistory.osu.edu/books/official-records/106/1096).
 The American Presidency Project, UC Santa Barbara (accessed May 10, 2022: https://www.presidency.ucsb.edu/node/202686
 U.S. Department of State, Office of the Historian, Historical Documents (accessed 5/9/2022, https://history.state.gov/historicaldocuments/frus1863p1/pg_659).
 The American Presidency Project, UC Santa Barbara (accessed May 10, 2022: https://www.presidency.ucsb.edu/node/203194).