The Western alliance promised to “hold Russia” and Belarus “accountable” for the “brutal and wholly unprovoked and unjustified” “invasion” of Ukraine on Friday, warning that Moscow would be made to pay “a severe price” for its actions.

Russian President Vladimir Putin ordered the military to put the nation’s nuclear deterrence forces on high alert Sunday following “aggressive statements” from NATO.

“Top officials of leading NATO nations indulge in making aggressive statements about our country. Therefore, I am ordering the minister of defence and the chief of the general staff to put the deterrence forces of the Russian army into special combat duty mode,” Putin said in a briefing with Defence Minister Sergei Shoigu and Chief of General Staff Valery Gerasimov in Moscow.

Putin’s order follows remarks by UK Foreign Secretary Liz Truss earlier in the day warning that if Russia’s military operation in Ukraine was not “stopped,” it could lead to a conflict with NATO.

“This long-running conflict is about freedom and democracy in Europe. Because if we don’t stop Putin in Ukraine, we are going to see others under threat: the Baltics, Poland, Moldova. And it could end up in a conflict with NATO,” Truss said.

White House spokeswoman Jen Psaki condemned the Russian president’s decision later Sunday, suggesting it was part of a “pattern” of “manufacturing threats that don’t exist to justify further aggression.” Psaki did not comment on Truss’s remarks.

Russia ‘Will Be Held Accountable’, NATO Says

The leaders of the Western alliance held an emergency virtual summit on Friday to “condemn in the strongest possible terms Russia’s full-scale invasion of Ukraine, enabled by Belarus.” In a joint statement, the alliance called on Moscow to “immediately cease its military assault, to withdraw all its forces from Ukraine and to turn back from the path of aggression it has chosen.”

The bloc warned that “the world” would “hold Russia, as well as Belarus, accountable for their actions,” and accused Moscow of bearing “full responsibility for this conflict” by “reject[ing] the path of diplomacy and dialogue repeatedly offered to it by NATO and Allies.”

The alliance promised to “take all measures and decisions required to ensure the security and defence of all Allies,” including through the deployment of additional land and air units in Eastern Europe and maritime assets “across the NATO area.” This has included the deployment of the NATO combat-ready response force ‘as a precautionary measure’, for the first time in the bloc’s history.

US media have also warned in recent days that a Russian cyberattack on Ukraine could trigger Article 5 -the NATO Treaty measure committing allies to joint defence in the event of an attack on one member, if such a cyber action impacts eastern Poland.

NATO chief Jens Stoltenberg warned Thursday that the alliance would protect “every inch” of the bloc’s territory, but noted that NATO does not have “any plans” to deploy troops in Ukraine.

“There must be no space for miscalculation or misunderstanding. We will do what it takes to protect and defend every ally, and every inch of NATO territory,” Stoltenberg said.

The NATO chief and others, including US President Joe Biden, have indicated that the alliance’s assistance to Kiev would continue include weapons and other support.

Ukraine Crisis: Decades in the Making

The current security crisis in Ukraine is at least in part a calamity of NATO’s own creation. Russian officials have spent years condemning the bloc for its decades-long eastward push toward Russia’s borders, and Washington’s unilateral moves to break security agreements with Moscow aimed at ensuring peace and strategic stability – such as the Anti-Ballistic Missile Treaty, the Intermediate-Range Nuclear Forces Treaty, and the Open Skies Treaty.

At each step, whether in incorporating new members into the alliance or ripping up a post-Cold War security treaty, the US and its allies continued to assure Moscow of their readiness to “talk” to address Russian security concerns.

“They will chat endlessly, speak endlessly about the need to negotiate, and do nothing, except pumping up our neighbour with modern weapons systems, and increase the threat to Russia, which we will then be forced to somehow deal with, somehow live,” Putin said in an interview with Russian media late last year.

In 2014, Ukraine’s bloc-neutrality-seeking government was overthrown in a violent, Western-backed coup d’etat, setting Kiev on a course for membership in the European Union and NATO. The coup led Crimea to break off from Ukraine and rejoin Russia, and sparked a civil war in eastern Ukraine between Kiev forces and local militias refusing to recognize the new regime. The bleeding wound in eastern Ukraine would continue to remain unhealed for nearly eight years, with Kiev ducking promises made to Germany, France and Russia to end the war by providing the self-proclaimed Donbass republics with constitutionally-guaranteed autonomy, in accordance with the 2015 Minsk Agreements.

In December, the Russian Foreign Ministry proposed two security treaties to NATO and the US aimed at easing tensions and restoring strategic stability to Europe. The draft agreements called on both sides to limit the deployment of troops, missile systems, aircraft and warships in areas where they could be considered a threat by the other side. Crucially, they also included a demand that NATO end its eastward expansion into the former Soviet Union, including Ukraine, and limit the deployment of forces in countries which joined the bloc after the end of the Cold War. Washington and the alliance openly and publicly rejected the Russian proposals, expressing hopes for continued talks.

Russia recognized the Donetsk and Lugansk People’s Republic as sovereign states last Monday. On Thursday, amid continued Ukrainian mortar and artillery attacks on Donetsk and Lugansk People’s Republic territories, Moscow launched a military operation in Ukraine which Putin said is aimed at “demilitarising and denazifying” the country.