The last nuclear treaty between the US and Russia is due to end in a matter of hours.
Technically, this will remove any constraint on the number of warheads either side can collect - raising the risk of a new arms race.
The New Start Treaty, an agreement signed in 2010 between Moscow and Washington to limit their respective number of nuclear arms, has kept each side to a cap of about 1,500 nuclear weapons and total stockpiles of around 5,000 weapons.
For some context - at the height of the Cold War there were as many as 70,000 nuclear weapons of all shapes and sizes between the two nations.
But unless Washington and Moscow reach a last-minute deal, the world's two biggest nuclear powers will be left with no limits for the first time in more than 50 years.
The exact time New Start expires isn't totally clear, with some believing it will run out at 11pm UK time tonight (midnight in Prague, where it was signed).
But what's for sure is this time tomorrow, the deal as we know it will have run its course.
What Russia says and signals from Trump
Kremlin aide Yuri Ushakov said Vladimir Putin discussed the issue in a video call today with Chinese President Xi Jinping, telling him Moscow would act "carefully and responsibly".
"We remain open to exploring avenues for negotiation and ensuring strategic stability," he quoted Putin as saying.
Donald Trump has given different signals on arms control, claiming last month that if the treaty expires, he would reach a better agreement.
But Russian officials say, so far, there has been no response from Washington on a proposal by Putin to extend the limits of the treaty beyond expiry.
A new arms race?
Matt Korda, associate director for the Nuclear Information Project at the Federation of American Scientists, said the deal's expiry doesn't necessarily mean the start of a new arms race, given the cost of nuclear weapons.
Our own military analyst Michael Clarke has a similar view, which he outlined in this week's Q&A.
"That probably won't change, because that picture is relatively stable and it suits them both," he said on the number of warheads stored by both sides.
The US has given no indication of renewing the treaty but might make a sudden decision "right up to midnight tonight", Clarke added.
In the meantime, it would actually be a good idea to take Putin up on his offer of an extension...
"Generally speaking, the Russians are always trying to make gestures to bring themselves back in from the cold, to make themselves acceptable again to the Western world.
"On this one, sheer pragmatism says if they're offering a rollover of an essential treaty, take it. Then you've got a year to sort out what we might be able to do about some of the other things."
"Conceptually, today is a big day to the end of the best regime for war limitation that the world has seen," Clarke concluded.