By Paul Reynolds BBC News Online world affairs correspondent

Gibraltar is one of those places left behind by the end of empire.

Gibraltarians rejected joint British-Spanish sovereignty

They are scattered across the globe, sometimes almost lost in the midst of oceans.

They are reminders of the past - and sometimes problems of the present.

Quite often, the controversies they generate go back as long as they do and their fate is determined by documents and decisions from centuries ago.

Hard to resolve

Gibraltar is particularly intractable.

Spain will not give up and Britain will not give in.

The new row, accompanied by the old insults, is over a visit by the UK defence secretary as part of the events marking the 300th anniversary of the capture of Gibraltar by a combined Anglo-Dutch fleet in 1704.

Spain wants Gibraltar back - lock, stock and rock. It lost it in the War of Spanish Succession, one of those incomprehensible wars of the 18th Century.

We had better not go back to the 8th Century when Tariq ibn Zaid conquered it for the Moors and gave it his name - Jebel Tariq, Tariq's Mountain.

The Treaty of Utrecht...

Spain argues that Britain could easily hand sovereignty back. The Treaty of Utrecht of 1713 says that if Britain gives up Gibraltar, it has to offer it first to Spain.

"And in case it shall hereafter seem meet to the Crown of Great Britain to grant, sell or by any means to alienate therefrom the propriety of the said town of Gibraltar, it is hereby agreed and concluded that the preference of having the sale shall always be given to the Crown of Spain before any others."

...versus the constitution of Gibraltar

However, Britain will not exercise that right because it has given a commitment in the 1969 Gibraltar constitution: "Her Majesty's Government will never enter into arrangements under which the people of Gibraltar would pass under the sovereignty of another state against their freely and democratically expressed wishes."

Gibraltarians themselves would like self-determination. Modern doctrines of decolonisation, not old treaties, should apply, they say. That of course could lead to independence, which is not on offer.

Her Majesty's Government will never enter into arrangements under which the people of Gibraltar would pass under the sovereignty of another state against their freely and democratically expressed wishes Gibraltar constitution

The Treaty of Utrecht clause, giving Spain the first option if Britain leaves, rules that out. This clause will be respected by both sides, despite some others in the treaty not being fulfilled. For example, the border with Spain is supposed to be closed and no "Jews or Moors" are supposed to "reside or have their dwellings" in Gibraltar.

And so Gibraltar remains, as a Spanish minister put it once, a "stone in the shoe" in relations between Britain and Spain.

Spain's territories overseas

It is not just Britain which has enclaves.

Gibraltar points to the Spanish footholds along the coast of Morocco, Ceuta and Melilla, as examples of how the wishes of the population in those enclaves take precedence over geography.

Spain does not accept the parallel of course. Morocco makes the occasional complaint - and even occupied one disputed island for a time not long ago - but does not press the point too hard.

British policy uncertain

Post-imperial Britain has never really made up its mind about its leftovers. It has offered to share Gibraltar with Spain - Gibraltar said no - and once proposed a leaseback plan for the Falkland Islands but the islanders also said no. A war with Argentina followed.

Britain even had to take the island of Anguilla in the West Indies back when the locals refused to be linked to neighbouring islands in independence.

Britain did hand Hong Kong back but there was a time limit in its agreement with China and when the time was up, in 1997, the agreement was honoured and more than, since technically the agreement covered the New Territories on the mainland only. Hong Kong's people (nearly 7 million of them) had no say.

Spain's own enclave of Ceuta in North Africa

The inhabitants of the Indian Ocean island of Diego Garcia were simply removed in favour of a major US air base.

Little Pitcairn, settled by the Bounty mutineers and their Tahitian companions, is currently embroiled in a row with Britain about whether British law even applies there.

French policy clear

France has had a more straightforward policy. It has simply incorporated its far-flung islands into the French Republic.

A few years ago, I took a ferry from Newfoundland to the French islands of St Pierre et Miquelon. After landing in the fog which often covers the Grand Banks, we found ourselves in what appeared to be Normandy. There were gendarmes, cafes, patisseries, even a new airport - the lot.

It is an example of how one country can have possessions very close to another's without undue difficulty, but it requires both sides to agree and Canada and France do agree. As do Britain and France over the Channel Islands.

The French also apply their principle in the Caribbean where the islands of Guadeloupe and Martinique are much more prosperous than their previously British equivalents like Jamaica. That is because they get French benefits. There is a price to pay - these places are not independent.

The really obscure ones

From time to time, some of these far-flung places will erupt into the news.

Others probably will not.

Who will worry that Baker Island in the Pacific is a Wildlife Refuge run by the US Department of the Interior?

Or that the French run Wallis and Futuna, whose Pacific island inhabitants celebrate Bastille Day as their national day?

And will anyone disturb Norwegian possession of Bouvet Island in the South Atlantic which is uninhabited and covered with glaciers?