Great Britain must have been sorely tempted to intervene in the US Civil War but feared extreme blowback if they backed a losing cause.
Many upper class Brits sympathized with the Confederates but the middle and working classes opposed slavery strongly. British and French imperialists hoped to profit from breaking up the United States. France had invaded Mexico and installed Maximilian as Emperor. The British government would have had great domestic difficulty in siding with the Confederates, as Louis Napoleon of France proposed.
https://en.wikipedia.org/wiki/French_intervention_in_Mexico