As his forces wore down the last of Confederate resistance around Petersburg, Virginia, General Ulysses S. Grant contemplated launching another invasion – this time thousands of miles to the west. His target was Sonora, Mexico, where a former California senator by the name of William M. Gwin had hatched an ambitious plot that threatened to pull several other nations into the vortex of the American Civil War.

In a confidential letter to one of his generals on January 8, 1865, Grant fretted over the “great danger” posed by Gwin, “a rebel of the most virulent order.” According to Union intelligence, Gwin had established a colony in Sonora, where he began attracting Confederate-sympathizing Californians. Grant believed that Gwin might use these forces to invade California itself and thus give new life to the Confederacy.

In such an event, Grant wrote, he “would not rest satisfied with simply driving the invaders onto Mexican soil, but would pursue him until overtaken, and would retain possession of [Sonora] until indemnity for the past and security for the future…was insured.” Mexico would thus become a new theater in the war, with truly global significance.

Gwin’s Sonora plan began not in Mexico, nor in the Confederate states of the South, but in the court of Emperor Napoleon III of France. On the pretext of securing payment for a national debt, Napoleon had invaded Mexico in 1861 and installed a paper emperor, Maximilian, by the spring of 1864. To move forward with any colonization of Sonora, Gwin therefore had to first convince Napoleon that the enterprise was worthwhile.

That he did. Working with French high officials and the emperor himself, Gwin detailed an ambitious plan to settle the mining district of Sonora with American prospectors and trigger Mexico’s version of the California gold rush. By early 1864 he had won the official approval of Napoleon and set off for Mexico to make this dream a reality.

As Gwin’s plan unfolded, Union and Confederate officials alike tracked his movements with growing interest. Although Gwin was acting independently and not as a Confederate agent, many now believed the future of the rebellion hung on his actions in Mexico. This was no mere mining venture.

To many Unionists, Gwin had achieved nothing short of a rebel coup in the court of Napoleon. Gwin may have professed neutrality, but his Mississippi plantation, his Confederate connections, and his previous record indicated otherwise.

As California’s leading politician through the 1850s, Gwin had established a reputation as a proslavery stalwart. Born in Tennessee, Gwin hewed the Southern line while serving California in the Senate, and he ensured that his fellow Californian congressmen often followed suit. When war broke out, his son enlisted in the Confederate cavalry, while his daughter moved to Richmond and became a Confederate belle. Gwin himself was arrested under suspicion of treason in 1861. He was released after a short confinement, but the stigma of disloyalty stuck.

Throughout the war, Unionist newspapers traced treason in Gwin’s every move. The Sacramento Union regularly blasted him as a “hoary-headed old traitor.” When rumor reached the Alta California of Gwin’s Sonora plan, the paper speculated on the prospect of a new rebellion rising “from the debris of the Southern Confederacy.”

General Irvin McDowell, commander of the Union’s Department of the Pacific, was equally apprehensive about Gwin’s recent movements. As war drew to a close in the East, McDowell assured Grant that his department remained on high alert to the emerging Confederate threat south of the border. McDowell even sent a force to southern Arizona as a safeguard against the feared invasion from Sonora.

Such vigilance was justified not only by Gwin’s reputation, but also by other pro-Confederate activity in the Far West. California harbored an active secessionist element, especially in the southern part of the state. McDowell’s predecessor in the Pacific department predicted that 32,000 “restless and zealous” secessionists stood ready to detach California from the Union at a moment’s notice.

Although no such force materialized, smaller insurrections periodically plagued California. In Los Angeles, Southern sympathizers – who probably constituted a majority in the city – paraded Confederate insignia and defiantly sang “We’ll Hang Abe Lincoln from a Tree.” Closer to Sacramento, a former member of Quantrill’s raiders launched a guerrilla campaign in an attempt to plunder funds for the Confederate treasury. The California raiders’ success was far from spectacular – a stagecoach robbery, a series of failed heists and two deadly shootouts – yet their actions deeply unsettled California’s Unionist population. This was merely a taste of what could be expected from the far larger threat in Sonora.

For their part, some Confederates saw salvation in Gwin’s plan. Leaders in the South understood that his colony would naturally attract like-minded rebels. Through Gwin’s diplomacy and Napoleon’s protection, the rebellion might gain a second lease on life, an escape valve in the West as the Confederacy’s prospects faded in the East.

Portrait of Napoleon III

Napoleon’s involvement was especially promising to Confederates. The French emperor did little to disguise his Southern sympathies, including turning a blind eye when his subjects sent money and munitions to the Confederacy. He may have stopped short of providing direct aid to the rebellion, but his support of a slaveholder in Mexico was perhaps the next best thing.

Predictions of Union disaster and Confederate revival ultimately came to naught. Although Gwin went to Mexico with Napoleon’s imprimatur, the newly installed Mexican Emperor Maximilian refused to follow through with the plan, rightfully fearing that Gwin would detach Sonora from his fledgling empire. Without the cooperation of Maximilian, Gwin was only able to attract a small cadre of would-be colonists to Sonora. And by the summer of 1865, he abandoned his plans and returned to the recently reunited U.S.

Upon his return, Gwin was arrested and taken under guard to Fort Jackson, Louisiana where he would spend nearly eight months in prison – a longer sentence than virtually any Confederate high official other than Jefferson Davis. Although Gwin repeatedly disavowed his Confederate affiliations, his prison term was proportionate to the anxiety he inspired amongst Union commanders.

While Gwin languished in prison, his old patron, Emperor Napoleon III of France, withdrew the last of his troops from Mexico. Mexican nationalists captured Maximilian in May 1867, and executed him one month later.

After his long imprisonment, Gwin fared much better. He eventually returned to California, where he became an active political powerbroker until his death in 1885, even helping his former Confederate son win election to the state senate. Although his search for Sonora gold failed, he launched several lucrative mining ventures in postwar California and established himself firmly within San Francisco’s social elite. Yet for some, Gwin remained a symbol of a more troubled time in the nation’s past, when civil war threatened to spill over international borders and the slave South reached into the courts of emperors.