Barack Obama`s claim that he is "president of the word`s oldest constitutional democracy" was either a deliberate insult to an ally that had just let him down or breathtaking ignorance on the part of a former president of the Harvard Law Review.

Leaving aside the question of how long some of its citizens had to wait for their democratic rights, the United States did not even acquire a draft constitution until 1787. A century earlier, England`s modern constitution had come into effect after the revolution of 1688 – although some would argue that the English had an ancient Gothic constitution even before the Norman conquest of 1066.

Explaining the UK`s largely unwritten constitution in his recent book The British Constitution: A Very Short Introduction, Prof Martin Loughlin says that if the subject of his book did not exist, it would rank as the shortest of OUP`s popular Very Short Introductions.

In another new book –The British Constitution: Continuity and Change, published by Hart as a festschrift for Vernon Bogdanor – Prof David Feldman devotes a chapter to what are called constitutional conventions. Explaining how these norms manage conflicts and tensions between political institutions, he notes in passing that constitutional conventions are vital to the election of the US president: the electoral college established by the US constitution bears no relationship to modern political realities.

Little more than a month ago, I reported here that an affirmative vote of both houses of parliament was not required for the government to use armed force overseas. There was, however, a constitutional convention that deployment decisions would be debated in parliament.

That analysis was based on a report by the House of Lords constitution committee. Presciently, the committee said it was "inconceivable that the prime minister would either refuse to allow a Commons debate and vote on a deployment decision or would refuse to follow the view of the Commons as expressed by a vote".

I reflected that view in my piece here last Wednesday, saying it would "be very difficult for any government to take military action if parliament had voted against it".

At that time, I thought it was unlikely that ministers would put a motion to parliament that they had any prospect of losing. I imagined that there would be some sort of anodyne "take note" motion that nobody could oppose. But that was not how it turned out. So where does this leave the UK`s constitutional convention on the use of armed force?

There must still be a parliamentary debate before any military deployment unless circumstances make this impossible. But one can argue that the convention also now requires a vote on a substantive motion and that an adverse vote would make military action impossible, at least until circumstances changed.

If this analysis proves to be correct, it represents a profound shift of power from the government to parliament. In evidence to the Lords constitution committee, the government made it clear that some ministers, at least, did not want to give parliament the power to approve or block military action. Those ministers seem to have shot themselves in the foot.

It`s one thing for a new constitutional convention to emerge in the United Kingdom. But what`s really extraordinary is that it seems to have spread to the United States. Under the US constitution, the president is commander in chief of the armed forces. We know that this gives the president authority to launch military action without prior approval from Congress since this is what he was planning to do last week.

As Obama said in his Rose Garden speech on Saturday, "I have decided that the United States should take military action against Syrian regime targets." Note the personal pronoun.

But he then went on to promise that he would "seek authorisation for the use of force from the American people`s representatives in Congress". Attempting to explain this apparent contradiction, the president said: "While I believe

I have the authority to carry out this military action without specific congressional authorisation, I know that the country will be stronger if we take this course and our actions will be even more effective."

Obama no longer seems very sure of his ability to launch military action without Congressional approval. Is prior approval now a convention of the US constitution too? The US secretary of state says it is not. But the president`s speech will certainly be cited as a precedent by those who may be opposed to future military deployments. And it would surely be as difficult for Obama to defy Congress as it was for David Cameron to defy parliament.

Even more extraordinary, the convention may even be spreading to France. As the Guardian reports, the French president is required by law merely to inform his parliament no later than three days after he has launched military action. Now, François Hollande is facing growing calls from opposition politicians for a prior parliamentary vote.

We know that Obama`s decision to seek Congressional support has been greeted with dismay in Israel. Political and military leaders in Jerusalem are reported to be horrified at the thought that Obama would now need to ask Congress before taking military action against Iran – and that Congress might oppose it.

I`m not so sure. If the threat to western interests becomes imminent, one would hope that Congress would understand the strategic imperatives. The legal basis for military action – collective self-defence – would be rock-solid, unlike the UK`s flaky "humanitarian intervention" arguments reported by me last Wednesday and confirmed, almost word-for-word, by the attorney general a day later.

And a future US military campaign might never get as far as Congress if it depended on urgency or surprise. Unusually, the action proposed against Syria requires neither. And since the wisdom of launching strikes against the Assad regime at this stage is clearly open to doubt, there is every reason for subjecting it to the political process.

But that does not mean that leaders of democratic states will always require prior approval for future military action. If presidents and prime ministers are sure they are acting in the best interests of those who elected them, they will still be free to act. What emerged over the weekend is, after all, just a convention.

law@guardian