Analysis: Can the UK legally attack Syria?
The note released by the government setting out its position on the legality of military intervention has the feel of a position formed, in part, by hard lessons learnt from the UK's involvement in the Iraq War. And it will divide top international lawyers.
Back in 2003, the big question that the Tony Blair government had to answer was whether British forces could legally invade Iraq alongside their American allies.
The problem was that the answer to that question was up for debate.
When the first version of the then Attorney General's advice was published in the run-up to the Iraq invasion, it appeared that he had no concerns about the UK's role.
He said that "authority to use force against Iraq exists" because Saddam Hussein had failed to comply with UN resolutions.
It later transpired that he had said something quite different behind the scenes.
The Attorney had argued that the safest legal course would be to obtain explicit UN authority to attack Iraq - but he said there might be justification for going in even if that authority never came.
Fast forward to today and the UK is facing a similar legal question: can Syria be attacked without explicit UN approval?
The government is seeking a resolution from the United Nations Security Council, which would make military intervention in Syria on humanitarian grounds lawful, but it is realistic about the chances of that happening because of the likelihood of a Russian veto.
And so with no resolution in place, the government is at pains to make clear that it would have the legal power to intervene - but that power is strictly limited.
The Syria note steers a course well away from anything that looks or smells like intervention to achieve a change of regime, or which might regard toppling President Assad as a valuable by-product of preventing chemical attacks. And it makes clear that that "humanitarian intervention" is legal only if it meets three onerous conditions.
Firstly, there has to be convincing evidence, generally accepted by the international community, of extreme humanitarian distress on a large scale, needing urgent relief.
This puts the evidential bar both high and wide. The evidence of atrocity has to be compelling and must persuade the international community as a whole.
Secondly, it must be objectively clear that there is no practical alternative to the use of force. But, most critically, the use of force must be directed solely at averting a humanitarian catastrophe and it must be confined to the "minimum necessary" to achieve that goal, and for "no other purpose".
So what's clear is that the legal note published by Downing Street, carefully limited in its language and scope, will divide opinion among international lawyers because, having set tough conditions for intervention, the government concludes that all three of them would clearly be met in this case.
The government also seems mindful of the critical distinction in international law and relations between legality and legitimacy.
The two are intricately linked. The military intervention to stop the attacks on ethnic Albanians in Kosovo in 1999 was legally questionable because there was no Security Council resolution permitting it. But at the same time, its legitimacy was widely acknowledged.
So the greater the sense of legitimacy, the fewer concerns of illegality.
The government will hope that its note will convince its critics that whatever the legality, there is moral legitimacy to intervene.