Was the Declaration of Independence Legal?

American and British lawyers squared off recently in a discussion over whether the Declaration of Independence was legal. The BBC reports as follows:

On Tuesday night, while Republican candidates in Nevada were debating such American issues as nuclear waste disposal and the immigration status of Mitt Romney's gardener, American and British lawyers in Philadelphia were taking on a far more fundamental topic.

Namely, just what did Thomas Jefferson think he was doing?

Some background: during the hot and sweltering summer of 1776, members of the second Continental Congress travelled to Philadelphia to discuss their frustration with royal rule.

By 4 July, America's founding fathers approved a simple document penned by Jefferson that enumerated their grievances and announced themselves a sovereign nation.

Called the Declaration of Independence, it was a blow for freedom, a call to war, and the founding of a new empire. This also, of course, answers the question of the why the South was not allowed to secede: Because they lost the Civil War.

It was also totally illegitimate and illegal.

At least, that was what lawyers from the UK argued during a debate at Philadelphia's Ben Franklin Hall.

(The rest of the article can be read here.)

It strikes me that this misses a crucial distinction: The Declaration was essentially an announcement that if certain demands were not met, the colonists would fight a war for their independence. Such things are not intended to be legal. No sane country is going to provide legal basis for its sub-regions to secede at will -- and as the British lawyers point out further on in the article, the US certainly didn't give it's Southern half that right under Lincoln. Instead, the colonists were making a last ditch appeal and (more realistically) an appeal for public and international sympathy as they prepared to fight a war of independence. If the British had won, the signers would probably have been hung as traitors. Given that they won, they are considered to be founders of the republic.

Rather than trying to put forward some theory under which the document was legal within the context of the British Empire, it seems to me that the correct answer is that the Declaration was legal by right of conquest -- an aged yet still apt concept.