Pale and tired after one of the most testing and sleepless nights of her premiership, Theresa May’s voice cracked just a little as she addressed journalists – and the British people – from the state dining room in No 10 Downing Street at 9am yesterday.

“There is no graver decision for a prime minister than to commit our forces to combat – and this is the first time that I have had to do so,” she declared with emotion etched across her face. “As always, they have served our country with the greatest professionalism and bravery – and we owe them a huge debt of gratitude. We would have preferred an alternative path. But on this occasion there is none.”

The conclusion that airstrikes would be carried out had been reached over several days of intense diplomatic activity between London, Washington and Paris. All three governments and their intelligence and military services had worked through several nights to agree a tripartite response to the killing of 75 people, including young children, in an apparent chemical weapons attack in the Syrian town of Douma a week before.

May’s cabinet had on Thursday unanimously backed the principle of airstrikes. But there were still crucial decisions to be made about their precise nature, purpose and scale, and, critically, about their timing.

For May, the need to act was beyond doubt – but the political risks were all too clear. Could a prime minister whose hold on power was so frail, in a country still traumatised by the decision, based on flawed intelligence, to go to war in Iraq in 2003, really authorise airstrikes without the backing of parliament? And without the support of the Labour party, whose leader Jeremy Corbyn was opposed?

Should she not wait until MPs returned to Westminster after the Easter break, which they do not do until Monday?

According to senior figures in Downing Street, it was at around 7pm on Friday evening, after the prime minister had been in a phone call with the French president, Emmanuel Macron, that the die was finally cast and the timetable set.

Macron had been as hawkish, and arguably more so, than May or even US President Donald Trump. The message was that he wanted to go ahead soon. The French president had been preparing the ground. On Friday morning, he had made a phone call to the Russian leader, Vladimir Putin, the only one of the trio to do so. In the call, Macron voiced his “deep concern” over the “continuing deterioration of the situation on the ground”, but also said that Paris wanted “to continue and intensify dialogue” with Moscow for a political solution to the Syria crisis.

If it was to happen, there had to be a balanced message to assuage Moscow. “The tone was direct, as it often is, but it was also constructive,” said one Elysée source, stressing that Paris now wanted to capitalise on the momentum of the strikes to bring Russia to the table in a renewed effort for a “political process” to end the Syria crisis.

No 10 had, throughout Thursday and Friday, been building the case for strikes and establishing their legality.

The attorney general had told May that action would not break international law. And the intelligence services had been sending in more and more evidence that the chemical weapons attack was, beyond reasonable doubt, the work of the Assad regime.

It was the kind of detailed evidence May needed if MPs were to protest in parliament that she had defied their will. Just before midnight on Friday, the prime minister, who was at Chequers, began a series of calls to UK political leaders, including Corbyn, Nicola Sturgeon and Vince Cable, to warn them of what was about to unfold.

She then stayed up until military officials were able to tell her that the airstrikes had achieved their aim and been a success.

She recorded a statement to be broadcast on television to the nation and then turned in. “She got a bit of sleep, we think,” said an aide. “Though not a lot.” At around 6am, she and her husband Philip were driven back from Chequers to Downing Street, where the prime minister was briefed by senior military advisers on the operations and the damage they had inflicted.

Inevitably there were difficulties in keeping all members of the tripartite alliance to the same script. Even before French fighter jets had returned to their bases on home soil from the Syria mission, the French defence minister, Florence Parly, said that Russia had been “warned beforehand” to avoid any confrontation or escalation. The Elysée quickly contradicted her and played this down, saying that standard “deconfliction” contacts had been made with the Russian military only once the operation had begun.

French sources said France’s part of the military operation had taken 10 hours from the time the country’s fighter jets took off to the moment they returned.

At her press briefing early yesterday, the prime minister was clear that the action, while aimed at the Assad regime, also carried a wider message to those who thought they could use chemical weapons on British, or any other country’s, soil. “We cannot allow the use of chemical weapons to become normalised – either within Syria, on the streets of the UK or elsewhere. We must reinstate the global consensus that chemical weapons cannot be used.”

On Monday she will address parliament, telling MPs why the action was necessary and taking their questions. Only then will she be able to fully assess the potential political implications of her first military operation as prime minister.