Lord Palmerston declared war on the United States six weeks after the tragic loss of the RMS Trent. The French Government followed suit only hours later.
The casus belli was the sinking of the British mail packet en route from Cuba to France. On-board were two Confederate diplomats, James Mason and John Slidell, envoys bound for Great Britain and France to press the Confederacy's case for diplomatic recognition and financial support for the Confederacy in the name of King Cotton.
The Federal Government considered Mason and Slidell to be contraband dispatching the screw frigate USS San Jacinto to frustrate the mission. While the San Jacinto raided the Trent, mysterious circumstances caused the RMS Trent to sink on November 8th with all hands lost.
Union Captain Charles Wilkes strenuously denied that the tragedy was the result of combat engagement, an incredulous explanation that fell on deaf ears in Richmond, London, and Paris. The United States government attempted to agree damages, but public opinion was too strong to ignore.
Over a century later as submersible technology was able to find the wreckage of the Trent, it was discovered that the disaster had been caused by a boiler room fire which sunk her within a matter of hours.
Addendum by Jeff Provine: The two European powers were already in North America dealing with the issues of the new Mexican government refusing to pay debts of the old. An Intervention had been agreed upon that October 31 with the Treaty of London, of which Spain also a party. Spain had already seized Veracruz by the time of the declaration of war on the United States, with troops from Britain and France following in the spring. The supply lines were already in place to support the South, who began the war with victories against the US.
The Treaty of London quickly fell apart as Spain and Britain refused to be part of France's anticipated conquest of Mexico. Nevertheless, the strength of the British fleet made it impossible for the United States to maintain its blockade strategy as outlined by General Winfield Scott's "Anaconda Plan" and supplies from France enabled the Confederacy to strike northward with naval assistance up the Potomac. With the fall of Washington, DC, in 1863, the United States itself had given up on the war as too costly and removed Lincoln from the presidency from its new capital, New York City, while other members of Congress holed up in Philadelphia wished to fight on.
The Confederacy consolidated its position and soon joined France in its expedition to Mexico as the French requested aid in 1865 when the war turned to the favor of the Mexican Republicans. Under the combined military of France and the Confederacy, Mexico became subjugated, beginning decades of Confederate imperialistic interest in Latin America. The smaller United States rebuilt itself on industrial might and pushed its attention westward to settlement. Peace between the two American nations would always be tenuous.