All is fair in love and war.
An interesting saying which:
According to Urban Dictionary means: http://www.urbandictionary.com/define.php?term=All%20is%20Fair%20in%20Love%20and%20War
And according to The Free Dictionary means: http://idioms.thefreedictionary.com/All’s+fair+in+love+and+war
I don’t think the term justifies deceit in general (refer to above definition). But deceit in the name of war and love? Well, that’s a tougher line to draw.
Any thoughts, dear reader?