During the Franco-Prussian War of 1870–1871 she was assigned to a squadron of French ships that attempted to blockade the Prussian ports in the Baltic Sea in 1870.

The Alma-class ironclads[Note 1] were designed as improved versions of the armored corvette Belliqueuse suitable for foreign deployments.

Unlike their predecessor the Alma-class ships were true central battery ironclads as they were fitted with armored transverse bulkheads.

[2] On sea trials the engine produced 1,676 indicated horsepower (1,250 kW) and the ship reached 11.99 knots (22.21 km/h; 13.80 mph).

[4] Thétis had a complete 150-millimeter (5.9 in) wrought iron waterline belt, approximately 2.4 meters (7.9 ft) high.

[7] During the Cantonal Revolution Thétis and her sister Reine Blanche spent much of September–October 1873 in the port of Cartagena, Spain where they could protect French citizens.

[8] She became the temporary flagship of Vice Admiral Roze after 31 October 1875 when the armored frigate Magenta caught fire and exploded in Toulon.

On 8 October 1885 she was commissioned as the flagship of Rear Admiral Marcq de St. Hilaire and sailed for the Pacific.

Thétis lost her propeller off Madeira and had to return to Cherbourg under sail where the admiral transferred his flag to the Champlain.