My opponent lost the moment he did not define "fight", "win", "Americans" and "Spartans" as well as not providing the situation and context he is referring to.
I think that the entire country of the United States would win a war against the Spartan society if they are alive today, however, I do not think that one American without any weapons would win a physical fight against 10,000 Spartans with weapons. He also does not define "win" -- "win" what? Is this an organized duel and only swords are allowed. As you have the burden of proof, I have won this debate by stating a case in which "Americans would lose in a fight against Spartans".
I realize that I am using semantics and totally misconstruing your true intentions but that's what you get for making a debate which you thought you had an automatic win in.
What makes you think that we would necessarily have guns? Do you have a gun in your hand? What if a spartan broke into your house, who would win? Your poorly defined who we're talking about and the specifications of the fight and thus have lost.