Landholding
"The Earth is Yahweh's, and the fullness thereof."
Psalms 24:1.
Principles
No men produced the land, so no men originally owned it or could sell it, so no men can buy it or own it.
All men always have a right to use any land without hindering others: a Right to Camp, Roam and Forage.
If I want to do what would hinder others' use of land, I need to compensate them, and we need consensus on that compensation. If we cannot reach consensus there is dispute; what I claim as mine, you claim as yours, as with any other dispute. I will not here explore the principles of dispute resolution.
Land Reseizure is justified where land has been seized, though this involves complications. I will not try to disentangle them; I will just mention the chief points that occur to me.
Unlike stolen movable goods, the land itself can neither be seized nor recovered. What has been stolen is (at most) the productive capacity of the land over the relevant period, and that is what should be reseized (or less, if the previous users would not have used the capacity fully).
He who sows deserves to reap, and he who improves land deserves to benefit. The value added to the land over the relevant period needs to be taken into account. Maybe not in full, but if someone steals a cent and turns it into a dollar by skilful investment, you probably would not demand the dollar, though you would rightly demand the cent with some interest.
As with any recovery of proceeds of theft, it is preferable to impose the loss on the thief rather than on a relatively innocent purchaser of his loot.
Recovered wealth needs to be distributed to the public. I see no obviously correct formula for this, but any redistribution is better than no redistribution!
(Henry George seems in general to have supported Producer's Right and Land Reseizure, but to have overlooked or undervalued some of the above complications. But Georgist readers are welcome to correct me.)
Europe
Once upon a time in Europe, most wealth was in land and was stolen, having been seized by the prevailing gangsters. Critics of private property, from the legendary Robin Hood's merry men (taking from the rich and giving to the poor) via Winstanley's Diggers to Proudhon's anarchists, advocated a righting of that wrong.
Accursed, who from the wrongs his father did
Would shape himself a right.
Alfred North Tennyson, Gareth and Lynette, lines 340-341.
Those rebels resisted not earned private wealth, only stolen wealth. In earned wealth, arising from enterprise and free trade, there has always been some dispersion. Those who work harder, or are more skilful, or more lucky, prosper more than others. Among those above-mentioned radicals, this was seldom raised and never pursued. The matter was too minor to bother with. I see no evidence that they would have seen it as a problem.
But the opportunity for redress of historic encroachments has largely passed. The gangsters have spent most of their loot, and most wealth is now earned. Galling for those keen to do justice, but there is no point flogging a dead horse. So those early radicals' complaint is far less salient than it was. Sadly, most of their contemporary admirers fail to understand this, and assume, without any evidence that I have seen, that their rejection of property rights would have included earned property.
True, a certain proportion of extant assets still derive from theft, so in principle I am open to a historic restorative land transfer. Perhaps a few ancestral estates could still confidently be assessed as stolen. Perhaps recent indiscriminate seizures from the rich, although aimed at earned wealth, may have restored the balance as regards stolen wealth. Perhaps the imbalance may now actually be on the other side, in which case strictly speaking some seizing back by the rich of wealth from the poor might be justified, but the rich can probably be persuaded to waive that. Let us seek a settlement, and thereafter let no group plunder another.
Elsewhere
Other continents have their own histories which may differ significantly from Europe's.
For instance, in some places current holdings derive not from seizure from previous settlers but by exclusion of aboriginal roamers, so the loss of produce may be less. (If they were not using it much they have not lost much.)
But the principles are the same, and I leave dwellers in those places to apply them to their cases.