The design is printed on an olive Gildan unisex shirt Ar15s and spartan helmets frame this spartan taunt "IF". The story that inspired the design goes like this...
After invading Greece, Philip II of Macedon sent a threatening message to the Spartans: "You are advised to submit without delay, for if I bring my army on your land, I will destroy your farms, slay your people and raze your city."The Spartans replied with one word: "If" 
Scholars are not sure if this story is true, but it is a good one.