From the strictly formal point of view, writes AJP Taylor in his Oxford History of England 1914-45, Britain's entry into the first world war was declared "as though King George V still possessed undiminished the prerogatives of Henry VIII".
A characteristically arresting thought. But this is just the start. "At 10.30pm on August 4 1914," Taylor continues, "the king held a privy council at Buckingham Palace which was attended only by one minister and two court officials ... The cabinet played no part once it had resolved to defend the neutrality of Belgium ... Nor did the cabinet authorise the declaration of war. The parliament of the United Kingdom, though informed of events, did not give formal approval to the government's acts until it voted a credit of £100 million, without a division, on August 6."
Nor was this the end of the prerogative powers. The governments and parliaments of Britain's dominions were not consulted either. Each governor general proclaimed the war on his own authority - except in Canada, whose parliament, alone in the empire, voted its approval. Over 300 million African and Asian people across the globe were thus involved, without any consultation, "in a war of which they understood nothing against an enemy who was also unknown to them", as Taylor puts it.
The past is another country. They do things differently there. Except, actually, they don't. Today it may be impossible to imagine many things about the first world war occurring in anything like the same manner. The suffering of the western front would not have lasted for a week in the age of the mobile phone or the human-rights lawyer, for example. But the prerogative power to take the nation to war, central to the process of August 1914, is still there almost unamended, as though nothing has changed since the late middle ages.
There is, of course, something of an exaggeration in this description. The royal prerogative powers, already formal in 1914, are even more formal today. Democratic politics, at least as much as human-rights law, has transformed the political context not just of a decision to go to war but of all the residual prerogative powers. In practice the government exercises these powers on the crown's behalf. And in practice it will only do so if parliament does not object. Nevertheless almost none of this is written down. In principle, if not in practice, a prime minister could take the nation to war in defiance of a parliamentary vote; he wields a theoretical power that a believer in the divine right of kings would have recognised.
Throughout the 20th century these powers remained mostly uncontroversial and unchallenged. Yet they persisted as the basis for war-making. In September 1939 the declaration was reported by Neville Chamberlain to parliament as an accomplished fact. MPs did get the chance to vote on substantive motions on the Korean war in 1950 and the Gulf war in 1991, but only after troops had been deployed and hostilities had begun.
The Iraq war in 2003 broke new constitutional ground in many ways, in particular because a British government for the first time allowed MPs a substantive vote on a war before it began. This was in one sense a tactical response to governmental weakness, an implicit recognition that domestic politics required Tony Blair to obtain the authority of parliament for an exceptionally divisive conflict. But it was also a response to a growing sense among constitutional reformers that the government's exercise of this most powerful of prerogatives was now under greater question.
Three months before the war, Blair debated this issue with the Commons liaison committee. He agreed with the MPs that it would be right for parliament, and good for government, for there to be a vote. "There is a right to vote," he even told the committee, before entering a caveat. "The question is, do you take that one step further and get rid of the royal prerogative. I do not see any reason to change it, but I do really think that in the end it is more theoretical than real, this issue." In practice, Blair said, "I cannot think of any circumstances in which a government can go to war without the support of parliament."
Arguably this was always the case. Yet it is important to recognise that the nature of modern warfare has significantly increased the realism of Blair's statement. It is difficult to foresee the kind of total existential threat to the nation that British governments believed they faced in 1805, 1914 or 1939. The wars that Britain is likely to fight in the 21st century are likely to be fought exclusively by professional forces, as part of an alliance, in the name of the international order rather than exclusively national interest, with real-time media coverage on both sides of the frontline, and in circumstances that cause minimal or no disruption at all to everyday life at home.
The longevity and adaptability of this country's old war-making rules have been truly extraordinary. But modern wars fought in modern democratic societies are increasingly posing new questions and demanding new rules, as the historic parliamentary vote in March 2003 over Iraq implied. This is not to advocate change for change's sake but to recognise that war-making in modern, rights-based societies requires new forms of legitimacy. Which is one of several reasons why Clare Short's private member's bill requiring parliamentary approval for military action is both so interesting and so important.
Short's bill requires governments to do what the Blair government did voluntarily in 2003. But by explicitly removing war-making from prerogative powers and vesting that power in parliament, the bill goes significantly further. The bill requires a government to set out a written political and legal justification of armed action. If parliament does not approve, troops must not be committed. While allowing governments to embark on armed conflict in emergencies, the bill also gives MPs retrospective powers over emergency deployments.
Short's sponsorship will inspire some and provoke others. But it would be petty to see the bill as an excuse to rerun old arguments about Iraq. For this is, in the best sense, a modernising measure. It seeks to bring both the law of war and the system of public accountability for war-making abreast of modern realities. It puts parliament in charge of the most important decision that any nation can ever take. Flaws the bill may have, but they can be ironed out in committee.
The bill's second-reading vote in October will be a defining moment for all MPs who claim to be modernisers. Already many big names in all parties have given their support. Not the least of those MPs was the late Robin Cook. If all those tributes to Cook mean anything at all then the vote on October 21 will be a chance to turn those words into something lasting and practical in his memory.