This post has been updated with Tillerson's comments Monday evening.

President Trump's uneasy detente with Russian President Vladimir Putin just became even more uneasy.

British Prime Minister Theresa May on Monday accused the Russian government of at least being complicit in an "unlawful use of force" on British soil after the poisoning of former Russian spy Sergei Skripal and his daughter last week. May gave the Russian government a day to offer a response, without which she said the attack would be regarded as an action “by the Russian state against the United Kingdom” and would lead to consequences.

The announcement represents a significant hardening of Britain's posture toward Russia, with which it has had a valuable intelligence-sharing relationship. Britain had been accused of a failure to stand up to Russia in a similar set of circumstances in 2006, when former Russian spy turned MI6 agent Alexander Litvinenko was fatally poisoned. Among those who were criticized then was May herself.

And just as the attack on the Skripals has tested May's willingness to stand up to Putin, so, too, would it seem to highlight Trump's reluctance to do the same.

The White House's first comments Monday notably declined to echo May's conclusion that Russia was at least complicit in the attack. At the daily briefing, White House press secretary Sarah Huckabee Sanders would say only that the United States stands with Britain, declining to address Russia's role and citing the need to sort through the details of the attack.

Hours later, Secretary of State Rex Tillerson had apparently sorted through those details, telling reporters Monday evening that the attack "clearly came from Russia."

But the question now is whether Trump will echo Tillerson and support the United States' ally and the very significant charges it's making. This is a president, after all, who has been willing to believe the best about Putin — and has often found enough of a gray area to do it. Trump has repeatedly questioned and undercut his own intelligence community's conclusions about Russia's interference in the 2016 election, and after a meeting with Putin a few months ago, Trump seemed to accept Putin's denials that Russia had done anything.

“Every time he sees me, he says, 'I didn't do that,' " Trump told reporters during a trip to Asia in November. “And I believe — I really believe that when he tells me that, he means it. But he says, 'I didn't do that.' I think he's very insulted by it, if you want to know the truth.”

Trump's main argument was that there are significant issues on which he wants to work with Russia, including the Syrian civil war. He has effectively suggested that Russian interference in a U.S. election is a sideshow — that it wasn't important enough to press Putin too hard on it. The upshot has been that Trump has done very little to challenge Russia.

This situation would seem to be somewhat different. We now have what Britain seems to regard as something not too far shy of an act of war — and something that could well draw an official government response. Yet we also have something on which Trump could be inclined to believe whatever denials Putin and the Russian government are likely to offer. If Trump has been skeptical of U.S. intelligence, after all, why wouldn't he be skeptical of Britain's conclusions? It's very easy to see Trump offering basically the same response he did in November, even if it further strains his already strained relationship with May and Britain -- and contrasts with Tillerson's comments. (You need go back less than a week to find Trump and Tillerson being on opposite pages on something hugely significant: North Korea negotiations.)

At the same time, Russia is the rare issue on which the Republican-held Congress has occasionally been willing to stand up to Trump. Last year, it passed new sanctions over Trump's objections. If congressional leaders press Trump, he may feel more compelled to repudiate Russia's alleged actions.

Or he may not. But if he doesn't, his lack of any real forceful response to Russia's provocations will become even more conspicuous.

Russian's foreign minister confirmed March 16 the country would expel British diplomats after Britain made a similar move for the poisoning of Sergei Skripal. (The Washington Post)