In an interview with Sky News, Mr Johnson put a brave face on his failure

It was Boris Johnson's coming of age as Foreign Secretary - a crisis moment in Syria, confrontation with the Kremlin and the great powers gathering to thrash out a solution.

He was tasked with coordinating them towards a tough common position that US Secretary of State Rex Tillerson could take to Moscow, having been criticised as Washington's poodle for not going himself.

But his plan to impose punitive new sanctions on Russia dramatically failed to take off.

Bullish as he arrived in Lucca yesterday, Mr Johnson said the US airstrike was a "game-changer" - a moment for Vladimir Putin to face a stark choice between backing "toxic Assad" or a return to the top table of nations.

:: Putin claims US is planning new missile strikes on Damascus


We've been here before on Syria but there's an opportunity

His plan to call for sanctions targeting Russian military officers involved in the Syrian conflict was opposed by the Germans, the Italians and even the French, and was absent from the declaration.

If the strategy was a carrot and stick approach to pressure Russia to withdraw its forces from Syria, there wasn't much stick for Mr Tillerson to take to Moscow.

In an interview with Sky News, Mr Johnson put a brave face on it.

He claimed it was a very successful meeting with a "great deal of agreement around the table" on sanctions - but legally, there should be a UN investigation first.

Tillerson: Russia either not serious or incompetent

The foreign secretary said Mr Tillerson could still make Russia a "powerful offer" - a way out of a damaging conflict, to fight IS terrorism with the international coalition and have their strategic interests in Syria respected.

He even predicted that the days of the Assad family holding power were numbered.

A source close to Mr Johnson said an investigation could narrow down targets for sanctions and that the message of condemnation was clear, adding: "We went into the G7 wanting unanimity on Assad needing to go; that Russia can play a role in combatting Daesh (another name for IS) and rebuilding Syria, and targeted sanctions including on military chiefs.

"We got all of those. Yes, we need more evidence to impose sanctions, but we are confident."

G7 leaders meet in Italy to lobby Russia over supporting Syria

A source at the G7 meeting told me the Italians "will never support more sanctions" against Russia, a key trade partner - particularly in defence - which has taken a knock from existing measures imposed after Moscow's invasion of Crimea.

The Germans were also said to be "cool" on the idea, according to a UK Government source, as they try to defuse confrontation to the east.

Did Mr Johnson simply miscalculate? Number 10 seemed to keep some distance from his approach.

Sanctions were not publicly endorsed by Theresa May, or mentioned during her phone calls to Donald Trump and the Canadian prime minister Justin Trudeau.

But Mrs May and President Trump said the meeting on Wednesday - in which Mr Tillerson is expected to say that Assad has to go and a handover of power to take place - was a "window of opportunity".

With more ammunition for Russia to exploit the West's differences on Syria, and play down Britain's influence internationally, it may be a window that's rapidly closing.