During the French Invasion of Mexico, Romero played a key role in lobbying the United States to oppose the French-backed Second Mexican Empire. When Abraham Lincoln and Secretary of State Seward appeared ambivalent on the matter, Romero supported the presidential campaign of John C. Frémont in 1864. Romero also supported mediation between the United States and the Confederacy in order that the United States could focus more on opposing the Mexican Empire. When the Civil War ended, he proposed direct military intervention under the leadership of either General Grant of General Sherman.