Editor's Note: Shashank Joshi is a senior policy fellow for international affairs on the Renewing the Centre team at the Tony Blair Institute for Global Change. He is also a senior research fellow at the Royal United Services Institute in London. The opinions in this article belong to the author.
(CNN) - As the clock ticks down to the UK's midnight deadline for Russia to come clean on its possession of advanced nerve agents, the government of British Prime Minister Theresa May is preparing an array of retaliatory options.
These range from diplomatic expulsions and economic sanctions all the way up to reinforcements of NATO and cyberattacks on Russian targets.
The UK's response will certainly be tougher than in 2012, when Russia was found to have fatally poisoned another former spy on British soil, Alexander Litvinenko, using similarly lurid means.
But one crucial factor in shaping the UK's options will be the level of support it can expect from allies, who may be less eager to pick a fight with Moscow.
The UK's first port of call is likely to be its closest ally in matters of defense and intelligence, the United States. British investigators are likely to have drawn on American expertise in tracing the nerve agent's Russian origins, and will have compared notes on the movements of known Russian intelligence officers in the run-up to the attack.
A punitive British cyber-attack against Russia will also benefit from close cooperation between America's National Security Agency and Britain's GCHQ, given the intimate collaboration between the two organizations.
The sacking of Secretary Tillerson, less than 24 hours after he expressed support for the UK, will cause deep concerns in London. But his successor Mike Pompeo is likely to be sympathetic to the British position, and President Trump has since said that if May were to point the finger at Russia, he would take that finding as fact.
Second, the UK will be looking to its broader allies in the NATO alliance. Theresa May's strong language on Monday, calling the attack an "unlawful use of force," suggests that she may invoke NATO's Article 4 clause.
This is less dramatic than the Article 5 mutual defense clause, which was used after the September 11 attacks, but it allows for consultations between allies if one member feels their security is threatened.
It has only been used a handful of times since NATO's founding in 1949, most recently by Turkey in 2015. NATO consultations would not only be a powerful symbolic gesture of solidarity with the UK, but they might also lay the ground for tougher alliance-wide measures, such as further British military deployments into Eastern Europe and the Baltic states, on top of those which have been made over the past several years.
NATO's northern and eastern members would probably welcome such a step, although its southern members, such as Italy and Greece, would be more wary.
Third, and more awkwardly, is support from the European Union -- a body that the UK is leaving, somewhat acrimoniously, next year. Both the UK and EU have repeatedly emphasized that their divorce should not undermine their mutual commitment to one another's security. And several high-profile European leaders, who otherwise spar with British politicians on a weekly basis, have come out with strong statements of sympathy.
But the real question will be how far the EU is willing to go. Its existing sanctions on Russia, imposed after Moscow's illegal invasion and occupation of Crimea in 2014, are already under pressure.
German firms have been hit badly, and senior figures in Germany's Social Democratic Party, which is re-joining a coalition government led by Chancellor Angela Merkel, have argued for sanctions relief.
Germany, Poland, Austria, Hungary, Bulgaria, and Slovenia all get a large majority of their gas from Russia. Persuading these states to further prune economic ties with Russia would be a major test for British diplomacy.
Fourth, and finally, the UK will want support from its wider partners. One important group will be the so-called Five Eyes intelligence-sharing network, comprising the UK, United States, Canada, Australia, and New Zealand.
Another will be the G7, a bloc that includes Japan, and which booted out Russia in 2014.
At last year's G7 summit in Italy, British Foreign Secretary Boris Johnson led a drive to slap sanctions on Russian military officers working with the Assad regime in Syria. That failed, but he may now resuscitate the idea at the next summit in June. To keep Russian President Vladimir Putin as isolated as possible,Theresa May is also likely to urge French President Emmanuel Macron to cancel his plans for a controversial visit to Moscow in May.
The attempted assassination of Sergei Skripal shows that Russia continues to tear up all the rules that shaped the post-Cold War security environment. Whether Moscow intended this or not, the attack on British soil has become a test of alliance solidarity in the tumultuous, uncertain shadow of Brexit and the Trump presidency.
The UK needs strong support from its NATO and EU partners, not only to shield itself from a Russian counter-response but also to demonstrate, clearly, that the West remains united, strong, and resolute despite the ferment of the past two years. If British leaders and diplomats can secure this support, they will be in a much stronger position to send a powerful message to Vladimir Putin.