The clock is ticking towards the British-set deadline of midnight Tuesday for the Kremlin to explain why a Russian developed military-grade nerve agent was used to poison a former Russian double agent and his daughter in a small cathedral town in south England.

No one in the British government is holding their breath for a Russian response — or an adequate one, from London’s point of view.

And as the deadline looms British officials are scrambling to garner the support of Western allies, and they appear ready, if necessary, to invoke NATO’s Article 5 on collective defense, which has only been invoked once before in the Western alliance’s 59-year history.

In her statement before a somber House of Commons Monday saying it was “highly likely” the Kremlin had authorized the March 4 poisoning of Sergei Skripal, a former Russian military intelligence officer recruited by Britain’s foreign intelligence service MI6, and his 33-year-old daughter, British Prime Minister Theresa May chose her words carefully.

She angled her words, say her aides, so as to be able to widen the international dimension of the unfolding political and diplomatic crisis that’s plunging Anglo-Russian relations to their lowest point since the Cold War.

“Should there be no credible response, we will conclude that this action amounts to an unlawful use of force by the Russian State against the United Kingdom,” May said in a key legal paragraph in her statement.

The law

Under international law, a state that’s been the victim of the unlawful use of force by another is allowed to respond. By carefully choosing her phrase, May indicated she is treating the attempted assassination of the Skripals not as a criminal act but as a state-sponsored attack on Britain as a whole.

That also prepares the ground for Britain to follow America’s example in the wake of the September 11, 2001 terror attacks on New York and Washington D.C. and invoke article 5 of the 1949 NATO treaty, which states: “The Parties agree that an armed attack against one or more of them in Europe or North America shall be considered an attack against them all” and that they should work together to ‘restore and maintain the security of the North Atlantic area.”

The language May used contrasts with the British reaction to the fatal 2006 poisoning of another former Russian spy, the dissident Alexander Litvinenko, who died after drinking tea laced with polonium. Then there was no talk of an unlawful use of force by Russia.

May has won words of general support from international allies, but none mentioned NATO’s article 5. U.S. Secretary of State Rex Tillerson said whoever had ordered the attack must face serious consequences.

In a statement, he said: “We have full confidence in the UK’s investigation and its assessment that Russia was likely responsible for the nerve agent attack that took place in Salisbury last week. There is never a justification for this type of attack – the attempted murder of a private citizen on the soil of a sovereign nation – and we are outraged that Russia appears to have again engaged in such behavior. We agree that those responsible – both those who committed the crime and those who ordered it – must face appropriately serious consequences.”

NATO’s secretary general, Jens Stoltenberg, said: “The use of any nerve agent is horrendous and completely unacceptable. The UK is a highly valued ally, and this incident is of great concern to NATO.”

After speaking with May on the phone, French President Emmanuel Macron said Paris stood in solidarity with Britain.

Skeptics

But British officials privately concede anxiety about what practical support they will be able to secure and the reception they’ll get when asking for a coordinated Western response to Russia. Officials welcomed the condemnation Monday of the poisoning of the Skripals from White House spokeswoman Sarah Sanders, but were left uneasy when she omitted to name Russia specifically as the culprit.

“Whatever measures Britain takes against Russia, what will really count is a realization from Washington to Brussels to Berlin, that a full scale strategy of the West is needed to show strength and resolve in the face of unacceptable behavior,” former British foreign secretary William Hague wrote in Britain’s Daily Telegraph Tuesday.

A former director of policy planning at NATO, Fabrice Pothier, says there’s little appetite among Europeans for adding more economic and financial sanctions on top of those already imposed — and renewed by the European Union this week for another six months— on Russia for the 2014 annexation of Ukraine’s Crimea. Relations between Britain and its European allies are being roiled also by the testy negotiations over Brexit.

Martin McCauley, a former London University professor and member of the Limehouse Group of Analysts, notes that European nations rebuffed British requests for a coordinated punishment of Russia for Litvinenko’s poisoning. “The Prime Minister is faced with a difficult problem,” he says. Putin may run the clock, he adds. “He will be hoping that the same thing happens as what happened after the Litvinenko affair. The EU basically did not want to know. This time the same thing may happen. Look at countries like Germany and Italy; they would like to soften the [current] sanctions so they can go back to normal business relations with Russia. They don’t want a conflict, they don’t want a row (dispute) with Moscow.”