America never lost a war? Care to retract that statement?
I didn't read the post that prompted your question, so whatever.
Anyway, I think it depends on what "lost" means in this context. In Vietnam the U.S. certainly lost a lot of blood and treasure to not accomplish its goals. Korea is considered a stalemate, but the success of keeping SK free from NK was certainly a worthy achievement. And while Iraq II was poorly conceived, executed, and managed, along with the loss of blood and treasure, the fact of the matter is that the U.S. gained access and control over the second largest oil reserves in the world. The question in that case though is whether it was worth it. Practically speaking, it remains to be seen.
And in traditional terms, the losing of a war means that the victor controls the government of the loser and possesses its territory and assets. The U.S. hasn't been invaded since 1812. So in that sense, one can look at Vietnam (the most obvious "loss" when discussing this issue) more as a failed war-like action designed to keep another government intact. Losses were certainly sustained, but America itself was untouched, its economy grew, and civil rights were expanded. There was much discontent about that war, but the net effect has been to prevent the U.S. from engaging in conflict on such a massive scale since then. And to this day, the U.S. is still the richest and most powerful nation on the planet. It's difficult to think of a country as losing a war when it only grew more powerful afterward.
Then there's the legalistic definition of "war." The United States isn't at war unless Congress declares it. While on first glance it might not seem to make any difference--that if certain actions are equivalent to war, then it's war. However, with a declaration of war, the United States' Federal Government gains plenary power in order to wage war. This means the taking over of the means of production to produce everything that's needed to achieve the desired outcome. It can suspend certain rights as well in order to bring what it needs to bear upon the nation that war has been declared upon. For example, in 1951 Truman seized the nation's steel mills in order to ramp up production for the Korean conflict, but was quickly put in his place due to the fact that Congress had not declared a war, and that without such a declaration, the executive branch could not seize private property for such use.
Now, let's look at at some countries who can, without debate, said to have lost a war. Take Japan in WW2. They lost in every sense of the word. In a less extreme example, Argentina lost the Falklands War. It seized territory and assets (mostly sheep) from the UK. Then the UK, desirous of recovering said sheep, set sail to the Falklands and gave Argentina the boot. Argentina took assets, controlled them, then lost those assets in an armed conflict, and to this day, the UK still possesses those assets.
Nothing of the kind has happened to the U.S.