"Even if the German's stole it, and it was recovered by someone in the US forces, our government gave written permission to that person to keep it, and formed a contract as good as a bill of sale, that made it legally theirs, and made it able to be owned or sold."
That's NOT the way it works, even under US law. Stolen property is recoverable at ANY point in the chain of possession, even if the current possessor bought it in good faith.
There is NO legal precedent for "Finders Keepers, Losers Weepers," not even if you thrown in a "Neener Neener Pork and Beaner" exigent circumstance.
In other words, the old school ground rights of possession don't apply.
"The gift which I am sending you is called a dog, and is in fact the most precious and valuable possession of mankind" -Theodorus Gaza
Baby Jesus cries when the fat redneck doesn't have military-grade firepower.