That is your key mistake.
America hasn't been in any wars since WWII. Furthermore historians "In general" agree on nothing, a sampling perhaps agree with your viewpoint. There are simply too many variables in the viewing, interpreting, and deconstructing of history for them to agree on anything solidly. If we (the public, including historians) had more up to date information on WWII, WWI, and all of the United States's previous wars, (that is to say, the same amount of information that we are receiving regarding wars since the popularization of the television) it is likely that "in general" people would view all wars as a loss. Because they are, all wars are a loss.
With that in mind, to say simply because more information is available concerning the conditions that America "Hasn't won a war since WWII" is slightly if not profoundly ignorant. The hard information (casualty reports and political documents) that said the USA won WWI, WWII, and many other wars says that they did in fact also win Korea, Vietnam, Afghanistan, and Iraq. The primary difference is that the public isn't looking at the same information that says we are winning, they are looking at wounded veterans and empty chairs where their loved ones sat. Statistically the US has won every conflict. Emotionally they have lost everything since the popularization of mass media.
That is the clincher. So don't "Are you kidding me?" 
Anyone with a reasonable understanding of contemporary American history and two brain cells to rub together would understand that.
Ah, clever move : )
1. About the war or conflict thing
Just because the congress hasnt declared any wars since WW2, does not mean that Vietnam, Korea etc were not a war. Just like if the chocolate chip cookies say there was no genocide that doesnt mean its true.
Just because AMERICA does not accept a country as a souvereign state (whysoever) does not mean, it is not one. JUST because the American government does not approve of the word war (with all its negative meanings) does not mean Vietnam etc were no wars.
2. "Winning or Loosing"
I will not elaborate on all the wars, the most recent ones should be remembered by you.
I) Vietnam war (note, its called vietnam WAR and not Vietnam CONFLICT).
America retreated from South/North-Vietnam and the communist forces overtook the south.
Please tell how the fuck is that a "win" in any kind?
II) Korea war. A armistice was signed, no win for both sides. No peace-treaty has been signed until now, so again -please- tell me, in which way do you think has this war been won by America?
Those two wars allready proove your argument of "political documents" wrong. Yes, there are political documents but in NO MATTER they state a win of America.
Your mass-media argument might be right in some aspects but regarding the question if the wars were won or not, it is totally invalid. (see political documents above)
Oh and please do not give me the 2 braincells shit.
