In societies which recognise private ownership and legally uphold it, theft is widespread but sometimes punished. Non-perishable goods which have been stolen are sometimes returned to their rightful owners - though often enough they pass into untroubled circulation, handed down in families or sold at auctions.
When such societies go to war with each other, as they do or did, different rules apply. The private possessions of enemy civilians are almost always regarded as legitimate objects of requisition or expropriation. Soldiers are traditionally rewarded for fighting successfully by being given permission to loot. (Sometimes they are also given permission to rape).
It doesn't much matter if your side wins or loses, the durable loot - stuff which hasn't been eaten or drunk - remains yours and can pass into untroubled circulation, handed down in families or sold at auctions. Even large and valuable objects are subject to this law of war.
When the Bolsheviks seized power in Russia, they expropriated the expropriators and filled vast warehouses with their Stuff. Through the 1920s and 1930s, they sold it off to raise foreign currency and had no trouble doing so - even objects with provenance which linked them to their original owners could be sold. The USA - founded on the right to life, liberty and the pursuit of property - was the best market for Bolshevik loot.
In World War Two, German Armies were looters on a grand scale, as were the civilian administrations established in occupied territories. When the railways weren't shipping Jews to the death camps, they were shipping Loot back to the Fatherland. Germany's top leaders expected to receive Loot by the train load, the Tribute of conquered lands.
At the end of the War, the Russian Armies entered Germany as invaders, looting whole factories, dismantling them and shipping them back to the Soviet Union. In comparison, looting by American, British and French forces probably did not rise much above souvenir hunting. But interesting souvenirs.
Despite well-publicised instances of the return of looted goods to the descendants of their rightful owners or to the public museums from which they were taken, most of the Loot of World War Two is being handed down in families and sold at auctions.
Going back in time, the same is true for the Trophies which British soldiers brought back from their many Imperial forays - these often stolen goods are now treasured heirlooms, reminding families of great great grandfather's adventures.
Wars and revolutions destroy lives and property. They also redistribute - across vast distances - goods and chattels, down to the smallest trinkets.
I'm a stamp dealer and I spend some of my time going through other dealers' boxes at stamp fairs and through cartons of stuff offered at auction. I sometimes wonder just how much of it has a clean history, an uninterrupted passage from one legitimate owner to the next.
The truth is that an unknown percentage of this stuff doesn't have a clean bill of ownership health. And, more interestingly, I am not sure there are very many people who think that it matters very much. It is as if the passage of time, at least for small items, washes things clean. After all, if you had to prove that you got it from Z who got it from Y who got it from X who .... then many businesses, honestly conducted and tax-paying, would simply grind to a halt.
Sometimes even those who have had things stolen lose interest in retrieving them. I recall a well-known philatelic Expert telling me of someone reporting to him the theft of a valuable item and providing the Expert with a photograph. Ten years later or thereabouts, the item duly appeared on the Expert's desk with a request for a certificate of genuineness. The Expert contacted the original owner and asked him if he wanted to take action. "Oh, I can't be bothered" he replied.