The dark shadow of a war involving America and Russia in Syria is looming large after public threats of missile attacks from President Donald Trump, Moscow’s warning of devastating counterstrikes in retaliation and hopes of negotiations fading away.

The bellicose rhetoric about a military answer to the crisis over the suspected chemical attack in Douma appears to have made the prospect of conflict almost inevitable, with the timing and scale of it seemingly the only issues yet to be settled.

The era of nations formally declaring war on each other has long gone, but Mr Trumps’s tweet threatening “Get ready Russia, because they will be coming, nice and new and ‘smart’” means that he will now find it extremely difficult, if not impossible, to back down from launching an attack.

The US president’s tweet was apparently made, unsurprisingly, without consulting any military commanders, or even his new superhawk national security advisor, John Bolton. Weary spokesmen at the Pentagon had a standard response to questions about it, saying: “I refer you to the White House to characterise the president’s tweet.” A key factor in military operations, the element of surprise, has disappeared from the planning equation.

The immediate reaction from the Kremlin was remarkably emollient with the foreign ministry observing that “smart missiles should fly towards terrorists and not elected governments”. But Mr Trump’s eruption had been triggered by Alexander Zasypkin, the Russian ambassador to the Lebanon, declaring: “If there is a strike by the Americans ... The missiles will be downed and even the source from which the missiles will be fired.” This implicitly stated that US and allied warships and bases from where missile launches had taken place will be hit.

Mr Zasypkin’s comments were made on al-Minar TV run by the Hezbollah militia, meant for a particular audience, and he qualified his talks of hitting back with the qualification that a conflict “should be ruled out and therefore we are ready to hold negotiations”.

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But Mr Trump had seen the initial remarks and fired off his tweets. Moscow’s further response later was that a US led military strike could be to destroy evidence about the chemical attack, which the Syrian regime claims was staged by rebels, a “provocation” to justify Western intervention. Maria Zakharova, the foreign ministry spokeswoman asked: “Is the whole idea to quickly remove the traces of the provocation [so] the international inspectors will have nothing to look for in terms of evidence?”

Mr Trump and his advisors are said to be considering a larger operation than the one in April last year when, in response to another chemical attack, involving sarin on the town of Khan Shekhoun, around 60 Tomahawk missiles were fired into the Sharyat airbase in Homs destroying a handful of regime planes.

The mission this time may target multiple bases and attempt to destroy, as well as chemical facilities, as many of Syria’s warplanes as possible – as well as command and control centres. This would also probably mean that the strikes are likely to extend over several days.

UK Ambassador to UN: Karl Marx would be ‘turning in his grave’ after Russia condoned use of chemical weapons on Syria

US Defence Secretary James Mattis said that the Pentagon stood ready to provide military options to President Trump. However, asked if he had seen enough evidence to blame Syrian President Bashar al-Assad’s forces for the attack, Mr Mattis said: “We’re still assessing the intelligence – ourselves and our allies. We’re still working on this.” White House Press Secretary Sarah Sanders added later that “all options are on the table” and that a “final decision” on action had apparently not yet been made.

The Trump administration has advised allies that it expects support for its actions. But there are doubts about the extent of that forthcoming. Emmanuel Macron was an early drumbeater for military action and Mr Trump’s first phone call, after the news came of the Douma attack, was to the French president to discuss punitive measures.

Mr Macron has said on coming to power that “if chemical weapons are used on the ground and we know how to find out their provenance, France will launch to destroy the chemical stocks.” He said on Tuesday that French action will be sticking to destroying chemical stocks. But these could be hard to locate. The regime of Mr Assad, the “Gas Killing Animal !” in Mr Trump’s tweet, will have plenty of time to hide and remove stock after the US President’s telescoping of the attack. It is unclear whether the French government would follow America in prolonged engagement, or drop out after the first round of bombing.

In Syria, regime forces and militias are reported to be evacuating airports and military bases which may be targeted and moving the more modern warplanes and advanced equipment to the Russian airbase in Latakia. Hezbollah fighters, meanwhile, were leaving the Tiyas airbase near Palmyra which was hit in an Israeli air strike on Monday.

Theresa May had to wait to speak to Mr Trump while he had two conversations with President Macron. The British prime minister had been initially against taking the military option before an investigation into the Douma attack, but is said to have come around to joining in during the course of the day.

But the cabinet is said to be divided on the issue, with Boris Johnson leading those who are keen for British forces to take part. Some senior Tories have pressed for Parliament to debate the issue before military action and Jeremy Corbyn warned that bombing would escalate to crisis.

The use of Turkish military facilities, especially the airbase at Incerlik, would be hugely advantageous to the US and its allies in a Syria mission.

The government of President Recep Tayyep Erdogan has taken a strong stance against the Assad regime and had trained and armed rebel forces. Immediately after the Douma attack, Ankara condemned the Douma deaths “as a crime against humanity” and stressing” the Syrian regime will have to pay a price.

But President Erdogan has been keen to build closer ties with Russia and Moscow has allowed Turkish forces to carry out operations against Kurdish groups inside Syria. And, following a phone call from the Kremlin to President Erdogan, Ankara ceased blaming the Assad regime saying, instead, that there should be a “careful investigation” before blame is apportioned. There will be no Turkish bases for Western forces to use in Syria.