I'm going with the first answer - an amphibious invasion of a country that is symbolized by a lion.
For the record, calling it "Operation Sea Mammal" or some variation thereof, is inaccurate, both because it has nothing to do with marine life, because it just allows noobish posts on the subject due to having no idea what we're talking about, and because there were two other operations in WWII named after sea mammals (Operation Walrus and Operation Delphine). Besides, there's no real point to renaming it - Barbarossa was just as poorly planned, and yet we refer to it by name.