At the time, it looked to many observers – including myself – that a new constitutional convention had emerged. Although legally the power to direct the armed forces remained part of the Royal Prerogative (and so exercisable without formal parliamentary approval), politically things looked different. Given the precedents set by Cameron, and a decade earlier by Tony Blair over Iraq, future Governments would likely choose to call deployment votes. Refusing to do so would be politically costly, and pointless unless most MPs opposed the policy – in which case a change of course would be more sensible.

In April 2018, however, Prime Minister Theresa May ordered an operation in Syria that looked very similar to the one vetoed in 2013. But she did not call a prior vote, citing the need for action during the Easter recess. Although she faced some criticism, MPs refused to censure her. The costs were minimal, in other words.

From comparing the 2013 and 2018 cases, what can we learn about parliamentary war powers more generally?

Reflecting on my article on this subject in Parliamentary Affairs in 2022, I think there are four key drivers of parliamentary involvement, namely: the view held by the Prime Minister of the day; party politics; cohort effects; and the nature of the proposed military action.

Cameron seems genuinely to have believed that he had a moral obligation to consult MPs before going to war. Despite defeat over Syria, he still asked MPs to approve operations against Da’esh in Iraq in 2014 and Syria in 2015.

May, by contrast, accepted no such moral obligation. On the contrary, she told MPs that she preferred a more traditional model of retrospective accountability:

“it is Parliament’s responsibility to hold me to account for such decisions, and Parliament will do so. But it is my responsibility as Prime Minster to make these decisions – and I will make them”.

More coherent parties – whose members broadly agree on the use of force – perform better in parliamentary votes than parties whose members hold a wider range of views. Larger parties do better, too, because they can absorb rebellions more easily. Some parties believe more strongly in the merits of military action, or of parliamentary oversight, than others.

Cameron’s decision to call a vote in 2013 partly reflected the fact that his Government was a coalition between the Conservative and Liberal Democrat parties. Although Conservative MPs generally support the use of force and reject parliamentary oversight, Liberal Democrats are more dubious of the merits of military action and more keen that MPs should have a say.

The debate over Syria in 2013 focused heavily on the question of whether the invasion of Iraq a decade earlier had been a mistake. Labour’s decision to oppose the Government motion in 2013, having supported intervention in Libya in 2011, reflected both political opportunism and the influence of memories of Iraq.

The debate in 2018, by contrast, dwelt on the consequences of non-intervention in 2013. Several speakers renounced their previous vote, declaring they had miscalculated.

Cameron never conceded that MPs should have a say in every situation. In August 2015, for example, he approved two drone strikes against Da’esh operatives in Syria without parliamentary consent. Explaining his decision, he cited the need to act quickly and in secret when taking such action against an imminent security threat.

Even before May’s action in 2018, most observers agreed that MPs should have the opportunity to veto major overseas military combat deployments, unless the emergency nature of the situation or the clandestine nature of the operation precluded open prior discussion.

May added a further caveat, arguing that Parliament should only be consulted where a large-scale deployment of ground troops was envisioned. Operations like those in Libya, or against Da’esh, would not be covered under that caveat, but only something on the scale of Iraq, the Gulf or the Falklands.

Moreover, if MPs approve of what the Government plans to do – or has already done – they tend to worry less about having a say themselves. Cameron faced little push-back for failing to consult before ordering his Syrian drone strikes. May was criticised in 2018, but the criticism was blunted by the fact that many of her critics supported the substance of her decision. It also helped that the operation was over, and nothing had gone wrong.

Because leadership, party politics, cohort effects and the nature of the specific action all affect whether MPs get to vote on military action, predicting what might happen in future is difficult.

The House of Commons Public Administration and Constitutional Affairs Committee (PACAC) concluded its 2019 inquiry into the issue by proposing that MPs agree a resolution clarifying the circumstances under which they expect to be consulted on the use of force. Nothing happened, in part because a non-binding resolution would not actually change anything of substance.

A future Government could decide to place Parliament’s role on a statutory footing, via a War Powers Act. Legislation would be harder to ignore than precedent or a resolution. But it would need heavy caveats to work in practice, and might risk transferring political discussions about the merits of military action from the House of Commons to the courts.

My preferred solution is simpler. Aspiring Ministers should recognise that consulting Parliament is a good idea:

• It is a good political idea, forcing potential rivals and opposition parties to take a public stand before an operation begins. Conservative support for the Iraq War in 2003 helped neutralise it as an election issue in 2005.

• It is a good strategic idea, especially if opposition parties vote in support, signalling to allies and enemies alike that a change of policy is unlikely.

• And it is a good policy idea. The best way to persuade MPs is to have a clear, coherent argument. Cameron’s defeat in 2013 reflected woolly strategic thinking as much as anything else.

Strong, J. (25 August 2023), Does Parliament decide on war? Revisiting the Syria vote 10 years on (Hansard Society blog)