The United Nations Charter, international law, English common law and, no doubt, common sense are as one: kill a couple of armed terrorists plotting unspeakable atrocities and you have acted in self-defence. You have saved lives.

It is necessary to take the Government's word for the plotting, of course, but such is the nature of a war fought in the shadows. Most will take it as given that if Reyaad Khan and Rahul Amin volunteered to fight for Islamic State they were prepared to do hellish harm to their country. The deaths overseas of two Britons at British hands might have no precedent. That doesn't make it illegal or wrong.

Nevertheless, we are entitled to see the advice, presumably from the Attorney General, Jeremy Wright, by which David Cameron and ministers set such store. We are also entitled to hear a lot more about government policy, whatever it now happens to be, where Syria is concerned. After all, Michael Fallon, the Defence Secretary, has said he “would not hesitate” to order RAF Reaper drones into action again. What does that mean?

The Commons rejected air strikes in the Syrian theatre two years ago. The Reapers, of which the United Kingdom seems to have 10, might be unmanned and operated from a base in Lincolnshire, but they are armed aircraft by any definition. How do their activities sit with what is – or was – policy? The RAF flies missions in Iraq at the invitation of the Iraqi government. Like it or not, there is no equivalent permission for Syria.

The United States ignores the difficulty. It interprets the right to self-defence widely. It deploys its drones as it sees fit, to meet its definition of a terrorist threat. The result since 2004 has been anything between 2,400 and 4,000 deaths, chiefly in Pakistan and Afghanistan, and continuing controversy, both over civilian losses and Barack Obama's use of “kill orders”. So a further question arises: is this now to be Britain's chosen tactic in the “war on terror”?

From Mr Cameron's point of view, it has advantages. First, the General Atomics MQ-9 Reaper is cheap enough. At around £10 million a craft, it is more economical, let's say, than a £125 million Eurofighter Typhoon. Drones also allow the Prime Minister to say, hand on heart, that he has not “ordered British personnel” into harm's way. The Whitehall mantra of “targeting” and “precision” can be employed. And the public can be relied on to believe that robotic war is somehow not real war.

A drone strike is still an air strike, however. Used in Syria, drones constitute military action within a country with whom – the technicality is also hard reality – no state of war exists. These “vehicles” have a habit, sooner or later, of killing civilians. The strike against Reyaad Khan and Rahul Amin might be an isolated case, but we don't know that. We do know that the Commons rejected a UK incursion into Syria. This was such an incursion.

Thanks to that marvel, the Royal prerogative, Mr Cameron does not require the permission of MPs to order the use of military force. There is, however, a modern convention that sees a vote as the nearest thing to the expression of a national consensus. The Government had plans to stage such a vote this month or next and secure support for conventional strikes against targets in Syria. Now a Reaper has been used in anger. If ministers “would not hesitate” to deploy drones again, what's the will of Parliament worth?

One way or another, Mr Cameron has circumvented the Commons even as the French, our partners in a drone development project, prepare to target IS within Syria. Some coincidence. The refugee crisis, equally, is fast being turned into an excuse for, in black comedy, “humanitarian war”. While agreeing to accept a trivial number of refugees – just 20,000 over five years – Mr Cameron can press on with his plan to tackle matters “at source” by attacking IS in Syria. Potential refugees should take heed.

The Prime Minister has meanwhile let it be known that if Jeremy Corbyn becomes Labour leader and “genuine consensus” is not forthcoming – that much is almost certain – plans for a parliamentary vote will be abandoned. The UK will creep towards escalation regardless. The killings of Reyaad Khan and Rahul Amin thus become the perfect preface for drone wars. Soon enough, the likes of Mr Fallon will be telling us that the difference between a Reaper and a Tornado bomber is so slight as to make no difference.

Clearly, no one is much dismayed by the deaths of two IS recruits. The prevention of a plot to cause harm at one of this summer's commemoration ceremonies, presumably involving the Royal family, will attract no protests. Drones and their effect on radicalisation might be another matter, if Pakistan is a precedent, but in this Mr Cameron surely has the consensus he seeks. He has acted to protect the country. What should interest us is the likely aftermath.

Ever since the Iraq debacle overseas interventions have been, as politicians like to say, problematic. The need to be disingenuous (we'll call it that) has increased, not decreased, since Tony Blair took us to war in 2003. There is no public appetite to see “troops on the ground”. Many MPs are more vigilant than before. But Downing Street (and the White House) still put its faith in military action. Drones are useful indeed.

Quite how the West will react when other countries employ these weapons to do away with enemies of the state has yet to be tested. No doubt our governments will denounce such assassinations. As with Iraq, they will find rival powers declaring that the right of the US and UK to lecture anyone is forfeit. We will also realise that the drone, so useful for keeping the losses down on your own side, has become the world's weapon of choice.

There is a suspicion, meanwhile, that Mr Cameron might not be too quick to abandon a Commons vote if Mr Corbyn becomes Labour leader. Disgruntled Labour MPs could be eager for an early chance to rebel against the new man. Would he dare to impose the whip, given his own voting record, and would it matter if he did?

In such circumstances, the Scottish National Party group is likely to hold firm. Brendan O'Hara, defence spokesperson, believes that with the drone strike “we are getting an insight into what may be going on”. The democratic decision of the Commons against involvement in Syria has already been flouted, in Mr O'Hara's view.

True enough. But the case of Reyaad Khan and Rahul Amin, together with the plight of refugees, has provided Mr Cameron with ideal cover. He will probably welcome opposition from the SNP when – as he will say – British lives are at stake. A drone would be cheap at twice the price.