So what will Russia do now? Will Vladimir Putin really reach for the red button?
In theory, he could. After approving changes to Russia's nuclear doctrine earlier today, he now has that option.
In what was a thinly veiled warning to the West, he lowered the threshold for a nuclear strike, allowing Moscow to respond with nukes if it's attacked by any state using conventional weapons. That includes missiles, aircraft, even drones.
Ukraine war latest: Ukraine launches six US long-range missiles at Russia, Moscow says
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Moscow now also formally considers an assault on its territory by a non-nuclear power (read Ukraine) that is supported by a nuclear power (read America) to be a joint attack.
It's not the first time we've heard these threats.
And now, after months of weighing them up, Washington is finally calling Moscow's bluff.
There are compelling reasons as to why Russia won't follow through.
Its main ally, China, won't tolerate it for starters.
More importantly, though, Moscow is unlikely to risk incurring the wrath of the new White House administration.
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The signs so far are that Donald Trump will stick to his promise and pursue a swift conclusion to the conflict, which would likely be in Russia's favour.
Read more:
Where do Russia and Ukraine stand militarily now?
Why UK missiles would only have marginal effect on Russia
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Launching a nuclear weapon now would almost certainly derail that.