On Tuesday, just hours after Ukraine appeared to have fired U.S.-made ballistic missiles into Russia — likely for the first time — the Kremlin published changes to its nuclear doctrine, lowering the threshold for what would justify a nuclear response.

The changes, which the Kremlin said were developed over several months, state that Russia will consider using nuclear weapons if its sovereignty is critically threatened by a conventional weapon with the support of a nuclear power.

"The enemy must understand the inevitability of retaliation for aggression against the Russian Federation;" said Dmitry Peskov, the spokesperson for the Kremlin, during a regular press teleconference on Tuesday.

An unnamed U.S. official told Reuters it was the first time the ATACMS were used by Ukraine inside Russia. 

Russian state media reported Ukraine fired six ballistic missiles at a facility in the Bryansk region overnight. Without offering evidence, RIA Novosti said Russia's air defence system shot down five of the missiles and damaged a sixth, which fell on the territory of the facility, sparking a fire that was extinguished.

Ukrainian media reported the missiles hit a weapons depot located 110 kilometres from its border with Russia.

An unspecified number of ATACMS (or Army Tactical Missile System) units were delivered to Ukraine last year. But the outgoing administration of U.S. President Joe Biden just reversed its policy and is now allowing Ukraine to fire the weapons deeper into Russia. 

Tatiana Stanovaya, a Russian political analyst and founder of R. Politik, an analysis firm, says Russia's decision to publish its nuclear doctrine on Tuesday was deliberately timed to respond to the ATACMS decision.

"This marks an extraordinarily dangerous juncture," she wrote CBC News over the messaging app Telegram.

"The current situation offers Putin a significant temptation to escalate. With Trump not yet in office, such a move would not interfere with any immediate peace initiatives, but could instead reinforce Trump's argument for direct dialogue with Putin."

'The catalyst of the escalation'

U.S. president-elect Donald Trump has repeatedly vowed to end the war in Ukraine quickly, without offering any specifics.

Stanovaya believes Russia calculated its response in an effort to try and deter Ukraine from using the missiles, and push blame toward Biden for "being the catalyst of the escalation."

Ukrainian officials and Western analysts believe the missiles, which have a maximum range of 300 kilometres, aren't likely to lead to a dramatic shift on the ground, because Ukraine has a limited number of weapons systems and Russia has redeployed many of its helicopters and bombers to airbases out of ATACMS range.

But according to the U.S.-based Institute for the Study of War, there are still hundreds of other Russian military targets in range of the weapons.

Ukraine received the ATACMS last October, and for the past several months it has pleaded to be allowed to use them more freely. Political observers believe the U.S. granted the permission now, because thousands of North Korean troops have been deployed to Russia's Kursk region for an anticipated counter-offensive.

In a surprise push in August, Ukraine captured an estimated 1,200 square kilometres in the Kursk region. 

Now, 1,000 days into Russia's full-scale invasion, its military, bolstered with North Korean recruits, appears positioned to try and take the territory back. 

"The situation is quite tense … the enemy is trying to oust us ... but at the moment, they are not able to," said a Ukrainian soldier who CBC News agreed to only identify as Wolverine, due to security concerns. 

Wolverine, who spent two months fighting in Kursk, spoke to CBC News from the Sumy region, where he was on leave but preparing to head back into battle. 

He described the recent ATACMS decision as "better late than never." But he says the fact it was a long time coming, and that it has been so widely reported, has removed any element of surprise. 

"The enemy can prepare and simply move its airfields, warehouses ... move its parts all to safe places where the weapons cannot reach."

Months of negotiations

Throughout the war, Ukraine's Western allies have grown increasingly comfortable supplying Kyiv with more powerful and deadlier weapons, despite concerns that it could escalate the conflict.  

Earlier in the war, there were negotiations over supplying Western-built tanks. Ukraine pleaded for F-16 fighter jets for months, before the jets started arriving this summer.

According to Matthew Savill, military sciences director for the London-based Royal United Services Institute, the impact of the ATACMS would have been greater had Ukraine been given the weapons and permission to fire them into Russia from the outset. 

In a written press briefing, Savill noted the Russian planes launching glide-bombs and cruise missiles at Ukrainian cities are largely stationed out of ATACMS range, with some parked 1,000 km away from the border. However, he says ATACMS would still be able target supply hubs, headquarters and ammunition depots.

"It may help the Ukrainians fight to hold on to the Kursk incursion … and inflict casualties on those North Korean forces that are now operating inside Russia," Savill wrote. 

"The impact [of the ATACMS decision] may be more political, albeit with a narrowing window of opportunity."

Ukraine's response

Speaking to reporters in Kyiv on Tuesday, Ukraine's President Volodymyr Zelenskyy didn't confirm the overnight strike, but said the country has ATACMS and "will use them, all of them."

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During the joint news conference with Danish Prime Minister Mette Frederiksen, Zelenskyy responded to Russia's ramped-up nuclear policy and urged Germany to supply Kyiv with Taurus long-range missiles, which it has been hesitant to do. 

"I think after statements about nuclear weapons, it is also time for Germany to support corresponding decisions," Zelenskyy said. 

France and Britain have not yet confirmed whether they will follow the Americans by allowing Ukraine to fire their Storm Shadow/SCALP cruise missiles into Russia. The missiles have a range of 250 km.

The Kremlin's response follows a familiar pattern — it's claiming that NATO was trying to escalate the war with a provocation that was akin to "throwing fuel on a fire."

The headline on the Tuesday morning broadcast of the state media news program 60 Minutes said, "If there is an attack on Russia territory, this means a direct attack by NATO."

Throughout the war, Russia has routinely accused NATO, and specifically the U.S., of waging a fight against Russia. 

"We are very much hopeful that Trump's people, and he himself, can overturn this decision," said Maria Butina, a Russian MP who spoke to CBC News over Zoom. 

Butina, who was convicted in the U.S. of being a covert agent, told CBC she believes the U.S. is close to pushing the conflict toward "World War Three," but would not comment on or even acknowledge that Russia has imported thousands of North Korean soldiers.

Reuters reported on Tuesday that Russia has begun serial production of mobile bomb shelters that can withstand a variety of threats, including shockwaves and radiation from a nuclear blast.

Butina says Biden's reversal on the use of ATACMS raises the stakes, noting that the U.S. and Russia are nuclear powers.

"While these big guys are playing politics in the United States, they don't fully realize that actually it could be the end for the whole human race."