Is Putin ready to reach for the nuclear button?

In theory, he could. After approving changes to Russia's nuclear doctrine on Tuesday, 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.

In short, US missiles fired by Ukraine meet the new criteria.

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.

We knew these changes were coming. Moscow proposed them in September.

But the timing of the presidential stamp of approval appears deliberate, coming just two days after the US gave Ukraine permission to fire American arms deep inside Russia.

It's unclear if Mr Putin was aware of the Bryansk attack or not, which Moscow claims involved six ATACMS ballistic missiles.

The Ministry of Defence issued its statement after he signed off the new doctrine. But whether he knew or not, he must have known it wouldn't be long before an American missile landed in Russia.

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.

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.

But there is always a chance, of course, that Russia could do the unthinkable.

After all, few predicted the full-scale invasion which started this crisis.

One thousand days on, the stakes are even higher, and the West is once again hoping it's interpreting the signs correctly.

-SKY NEWS