To win the day means to be successful, so it was clear that the asps must be victorious.
Consider the statement "It was clear that the asps must be victorious, though-and then the teal had struck heavily aground, throwing everyone flat on deck.". Is the statement "It was clear that the asps must win the day, though-and then the teal had struck heavily aground, throwing everyone flat on deck." false? Yes/no/unknown