Did they pull yer leg?
Let me put it this way:
To seek to gain the upper hand by surprise when starting a war against a foe fully able to defend itself against you is sound strategy. To do so against a realm already beset by a foe significantly larger than itself, one whose strength you outmatch on your own by a fair amount, stains your reputation to no good purpose.
But to advance upon such a realm, be challenged by its ruler, and respond with a ridiculously transparent fabrication, then go through with the attack? Why would you ever do such a thing? What can you gain by it but the contempt and distrust of your peers?