“We clearly need a War Powers Act in this country to transform a now-broken convention into a legal obligation”

That’s what Jeremy Corbyn told the Commons on Monday after Britain, France and the United States launched air strikes aimed at targets in Syria over the weekend.

The strikes came after a chemical attack on the city of Douma in the southwest of Syria on 7 April, which Mr Corbyn condemned in his statement.

But he also criticised Theresa May for engaging in the strikes without consulting Parliament. Labour are now calling for legislation that would force the government to get MPs’ approval before military action.

But would Labour’s proposed War Powers Act actually work?

FactCheck takes a look.

What is the current legal position?

“The power to commit troops in armed conflict is one of the remaining Royal Prerogatives – that is powers that are derived from the Crown rather than conferred on them by Parliament”, writes Dr Catherine Haddon, senior fellow and resident historian at the Institute for Government.

This means that “there is no codified parliamentary procedure that formally requires the Government to seek approval before taking military action.” In other words, in legal terms, it’s up to the Prime Minister and the Cabinet to make the call.

Of course, governments are still political animals, and since 2003, it’s been an unwritten rule of foreign policy that MPs have a say on any significant military interventions. But even if a vote is held, the government is under no legal obligation to abide by the result.

But after the strikes over the weekend – on which Parliament was not consulted beforehand – Labour says it’s now time to turn that convention into a properly codified War Powers Act. The proposed Act would “enshrine in law that the government must seek parliamentary approval before committing to planned military action.”

Would a War Powers Act work?

Let’s start by looking at what Labour say they want to achieve.

In a statement, the party said: “In the most serious matters of peace and security, the Prime Minister of Britain should be accountable to Parliament, not to the whims of any other governments.”

It’s not the first time a major political party has proposed such a policy.

The former foreign secretary, William Hague, now Lord Hague, told Parliament in 2011 “[the Coalition government] will also enshrine in law for the future the necessity of consulting Parliament on military action.”

But that legislation never materialised. Why?

Speaking on Radio 4’s Today programme on Tuesday, Lord Hague said that he had “reluctantly” changed his mind after discussions with the Attorney General and the Defence Secretary. He concluded it would be “very, very difficult” to pass such a law “in a way that was compatible with the safety of our armed forces and the maximum security of the country.”

He said: “it is quite difficult to explain [the details of a mission] to Parliament without, of course, revealing all of that information to people who you might not want to know that and people who you might want to keep guessing about how extensive that action will be.”

This issue remains a serious practical concern for Labour’s proposed War Powers Act.

Imagine the government is putting the case for a planned military intervention to Parliament. They could release sensitive information that may convince Parliament of the case for action – but in so doing, they might endanger the very operation they wish to carry out.

Alternatively, they might hold back the most sensitive details for security’s sake – even if they were the most compelling – but risk losing the vote in the process.

This could leave the government of the day in a potentially paralysing dilemma: they can’t act without parliamentary approval, but they can’t put the vote to Parliament without damaging to their own mission.

One way round this is to provide exemptions for different types of military action – for example, interventions that require the element of surprise or that need to be conducted very quickly.

But in practice, this risks rendering the law pointless. The United States took this approach with its own War Powers Act in 1973, which has since been regularly ignored by presidents of both parties. Indeed, President Trump’s recent strikes on Syria were not ratified by Congress in advance.

Dr Haddon sheds some light: “the key point is that War Powers Acts would cover certain kinds of military action, but not all.

“There would still have to be some caveats for urgent or necessarily secret action. The US [War Powers] Act has all these problems and they usually mean Congress does not have an effective vote in most circumstances.” Nevertheless, the US War Powers Act gives Congress members and campaigners something to point to when presidents chose to take unilateral action.

So Labour has two options: a War Powers Act that requires all types of military intervention be put to a parliamentary vote with no exceptions; or a War Powers Act that allows for exceptions in certain circumstances.

In order to decide, they must must weigh up three factors: security, timeliness and accountability.

Option 1: A War Powers Act with no exemptions

If the War Powers Act requires that all types of military intervention must be put to Parliament with no exceptions, it could force the government to reveal information that undermines national security.

It would also limit the government’s ability to respond to a very urgent threat (for example, shooting down a missile).

However, it would be an effective way of ensuring that Parliament can hold the government and the Prime Minister to account on military matters.

Option 2: A War Powers Act with some exemptions

It would be theoretically possible to draw up a list of criteria in a War Powers Act that, if met, would allow the government to take military action without a parliamentary vote.

That would help allay fears over revealing information with national security implications. It would also allow the government to respond to very time-sensitive situations.

However, as we’ve seen in the US, this would limit the ability of Parliament to hold the government to account over military matters. Or at the very least, it could leave us in exactly the same position as we are currently in.

Do you actually need legislation to give Parliament these powers?

And it’s not clear that a War Powers Act would always be necessary, if the point of the exercise is to increase Parliament’s ability to block military action that it disagrees with.

We spoke to Vernon Bogdanor, Professor of Government at King’s College London.

He pointed out we should “distinguish between an intervention – as with Iraq, Libya, or the proposed Cameron intervention in Syria in 2013 – for which it may be reasonable to ask Parliament’s approval; and a kind of police action such as occurred last week [in Syria] – a limited attack or retaliation which may need the element of surprise, and which does not involve intervention or the use of ground troops.”

Professor Bogdanor pointed out that “although going to war is a prerogative act [i.e. within the existing powers of the government], Parliament can always stop something which is more than a police action by withholding supply [of money].”

He uses the example of the First World War: “If, for example, Parliament had been opposed to war in 1914, it could simply have denied money for the armed forces.”

Professor Bogdanor adds: “But, if you look at our 20th and 21st century interventions, all of them, I believe, were supported by the Opposition as well as the government – except for Suez – one reason perhaps why it failed. The Opposition harried the government in the Commons day after day, and made life extremely difficult.”

In other words, Parliament already has the practical means to stop a large-scale military intervention: it can turn off the money tap.

What it is less equipped to prevent is the kind of time-limited, one-off “police action” (to use Professor Bogdanor’s term) that the UK conducted in Syria over the weekend. Clearly, this is the kind of intervention that Labour have in mind in proposing the War Powers Act.

FactCheck verdict

Labour are proposing a War Powers Act to require the government to “seek parliamentary approval before committing to planned military action.” They say this will help make sure that the Prime Minister is “accountable to Parliament.”

But Labour are faced with a dilemma. Do they insist that the law covers all military interventions, or do they allow exemptions?

If they chose the first option, the government would almost certainly become more accountable to Parliament on military matters. But, by that same token, the government could be forced to reveal information that could threaten national security. They’d also be unable to act even in the face of a very urgent threat (for example, shooting down a missile).

However, if Labour go for option two and allow exemptions to the War Powers Act, they face undermining their whole project to improve accountability. In practice, as we’ve seen in the United States where the War Powers Act is regularly ignored by presidents, exemptions would do very little to improve parliamentary scrutiny of the government.

It’s also not clear that a War Powers Act would even be necessary to help Parliament have a say over significant military interventions. Parliament already has the ability to block major military action (on the scale of a full-blown war) by withholding funding for the armed forces. That said, it is less well-equipped to block the short-term actions that we saw over the weekend in Syria.