Explanation: The Roman Emperor Caligula once ordered his troops to 'make war' on Neptune, the Roman god of the sea, by stabbing the incoming tides and collecting seashells as booty. Interpretations of this act... vary. Some attribute it to Caligula's supposed madness - others, to him having a bit of a lark by exercising how total his control as Emperor was ("I can make you lot do anything, no matter how stupid or humiliating" sort of thing, very popular amongst tyrants of all ages). Some say it was to humiliate the troops for refusing to go on a supposed campaign to Britannia, which was not yet conquered at that point.
We know the truth, though - that bastard Neptune was getting too uppity for his own good, and Caligula saved us all from a future under the sea god's oppressive reign!