Hundreds of thousands of NATO troops will be switched to high-readiness alert following Russia 's invasion of Ukraine.
Alliance Secretary-General Jens Stoltenberg revealed more than 300,000 forces will be stood up to the category - a huge boost from the 40,000 in the coalition’s existing response force.
The announcement came as NATO leaders prepare to descend on Madrid for a crunch summit tomorrow.
Speaking at a press conference at NATO headquarters in Brussels, Mr Stoltenberg pledged a “fundamental shift in NATO’s deterrence and defence”.
He said: "We will transform the NATO response force and increase the number of our high readiness forces to well over 300,000.”
The measure is designed at bolstering defences on the 30-member alliance’s eastern flank, where Baltic states border Russia.
The countries fear mounting aggression from Moscow following Vladimir Putin ’s war on Russia’s neighbour.
Finland and Sweden’s bids for NATO membership are due to be discussed when coalition leaders, including US President Joe Biden and Prime Minister Boris Johnson, take part in negotiations in the Spanish capital.
Leaders are also expected to overhaul NATO’s whole approach to the Kremlin.
The alliance's last strategy from 2010 described Moscow as a strategic partner - before it annexed the Crimea in 2014, supported anti-government rebels in eastern Ukraine in the same year, and launched a full military invasion of Ukraine on February 24.
Mr Stoltenberg confirmed: "I expect that allies will state clearly that Russia poses a direct threat to our security, to our values, to the rules-based international order.”
Earlier, Boris Johnson insisted the “price of freedom is worth paying” as he compared Russia with Nazi Germany.
Speaking at the G7 Summit in Bavaria, the Prime Minister vowed to continue supporting Kyiv and President Volodymyr Zelensky, adding: “Strategic endurance is very important.”
Hitting out at the “absolutely unbridled aggression” from the Kremlin, he told the BBC : “I think that sometimes the price of freedom is worth paying and, just remember, it took the democracies in the middle of the last century a long time to recognise that they have to resist tyranny and aggression.
“It was very expensive but what it bought in the end with the defeat of the dictators, particularly of Nazi Germany, it bought decades and decades of stability, a world order that relied on a rules based national system, and that is worth protecting, that is worth defending - that delivers long term prosperity.”
He said the economic impact on Britain of the war in Ukraine - which has seen fuel costs soar and food prices rocket - “will start to abate”.
He added: “We'll find ways around things and some of the cost pressures will start to come down.
“In terms of staying the course, imagine if we allowed Putin to get away with the violent acquisition of huge chunks of another country - sovereign independent territory.
“The lessons for that would be absolutely chilling - in all of the countries of the former Soviet Union, and see what's happening in the Baltic countries already.”
Ukrainian President Volodymyr Zelensky urged the G7 for more weapons to repel Russian invaders. Speaking via video link after Kremlin missiles rained down on Kyiv,
He called on them to be "partners, not observers" and provide the military equipment his country needs.
