Tony Blair opened the speech that would launch Britain into war with Iraq with these momentous words: “It is right that this house debate this issue and pass judgment. That is the democracy that is our right but that others struggle for in vain.”
Ever since that fateful day in 2003, MPs have jealously guarded the right to authorise military action. In 2006, it was endorsed by a House of Lords constitutional inquiry and enthusiastically advocated by the future foreign secretary William Hague in 2007.
Limiting the royal prerogative, on which matters of war and peace have always relied, was taken as one of the tokens of a modernising Conservative party. In 2011, the coalition government pledged that it would observe the convention, except in circumstances of an emergency.
Yet a vote in parliament was still a negotiable affair. There was no debate before the significant escalation of British involvement in Afghanistan in 2006, and approval for involvement in Libya in 2011 was only asked for, and given, after it had begun.
Legal experts pronounced the so-called convention to be more of a trend than a substantive change.
In 2013, however, David Cameron sought approval for a punitive strike on Syria in circumstances very similar to Friday night’s, and on broadly the same legal basis. His defeat – or at least, the fact that he accepted it – appeared to mark a turning point. It was considered to have greatly strengthened the power of parliament.
The following year, again observing the convention, Cameron asked for approval for intervention against Isis in Iraq. This time parliament approved the motion.
It appeared that the convention that parliament has a say in the deployment of military force was meeting the first requirement of any convention – that it is respected by those who technically have the power to ignore it. The outstanding debate appeared to be only on when it was permissible to ignore it.
That makes this morning’s attack by Penny Mordaunt, the international development secretary, so significant. (See 8.52am.) It is highly likely her position – that the intelligence on which a decision such as Friday night’s was based cannot be shared with MPs – will be echoed by the prime minister this afternoon.
Opposition MPs, who have been demanding the recall of parliament ever since the chemical strike happened more than a week ago, will be furious. There are two significant reasons why May could have misjudged her strength.
The first will be that MPs – possibly including some on the government side – will fight a straightforward defence of a hard-won power against its erosion by a government that has already shown scant regard for parliament in Brexit negotiations.
The second is political rather than constitutional. The SNP leader at Westminster, Ian Blackford, has repeatedly argued that a minority government like May’s has a particular obligation to consult parliament and carry it in order to have legitimacy for its major decisions. That is also a case that will be made today.