NATO looks set to agree next week to the biggest overhaul of its defences since the Cold War, including a major expansion of a 40,000-strong force on alert to respond in a crisis.

Russia’s invasion of Ukraine – which is not in NATO – has triggered a fundamental rethink in how the alliance deters President Vladimir Putin from attacking any of its 30 member states, according to interviews and briefings with half a dozen military officers and diplomats.

Long-standing defence plans that had not received much traction prior to 24 February – the day Moscow launched its all-out assault on Kyiv – are starting to become a reality as leaders meet for a landmark summit in Spain from 28-30 June.

“Overnight the mentality changed,” said a NATO military officer. “NATO now feels like it is electrified. You can feel the energy surging through the system.”

A diplomat predicted that the Madrid summit will deliver – or at least agree the framework to deliver – “a radical change in posture”.

Ukraine news live: Putin’s authority in Russia ‘eroding’

There is disappointment though that a desire by historically neutral Sweden and Finland to join the alliance – in the ultimate snub to Mr Putin and win for the allies – has yet to be activated because of concerns by Turkey. Ankara has accused Stockholm of harbouring Kurdish militants. The UK is one of the allies trying to resolve the rift in time for Madrid.

Among the steps expected to be given the green light are:

• An expansion and rebranding of the 40,000-strong NATO Response Force, possibly by as much as six-fold, according to two military sources. But a NATO source said various formulas are being discussed and the final increase would likely be lower.

• An uplift in the size of a mission to deter Russia from targeting NATO’s eastern and southeastern flank, with thousands more troops added, though many are set to be based in their home nation and only deploy forward if needed.

• The designation of Russia as the “most significant and direct threat” to security.

• A new “comprehensive assistance package” for Ukraine, including equipment to counter Russian drones and provide secure communication.

Jens Stoltenberg, the NATO Secretary General, said the gathering of heads of state and government in the Spanish capital will be a “transformative summit because we are at a pivotal time for our security”.

Evidence of the horror unleashed by Russian forces in places like the Ukrainian town of Bucha appear to have hardened the resolve of all member states to agree to strengthen their defences in a way that would have been unthinkable even a year ago.

A NATO defence minister is understood to have told his counterparts at a meeting at the alliance’s headquarters in Brussels last week that a Bucha-style atrocity cannot be allowed to happen on NATO soil.

If Russia tried to invade it ‘would be wiped off the earth’

This changed appreciation of the threat is changing NATO’s policy on how to deter it.

The allies look set to move away from relying purely on the threat of punishment being enough to stop President Putin from acting, to what some are calling “deterrence by denial”.

Kusti Salm, the top civil servant at Estonia’s Ministry of Defence, explained it would work by having troops in larger numbers already on the ground to stop any invasion before it started.

“You are able to put up adequate, sufficient forces and you indicate to your potential adversary that the force-overmatch is on a level that you [the adversary] would lose immediately,” he said in a briefing with journalists.

“This message should translate into a loss of their willingness to invade… Russia can read from it that even if they tried they would be wiped off the earth in the first few hours.”

Forces mobilised within days

The new thinking requires NATO allies to have even more combat-capable soldiers, sailors, marines and air crew ready to move at varying degrees of readiness. This can range from two days’ notice to mobilise to six months.

The NATO Response Force, or NRF, already offers US General Tod Walters, the Supreme Allied Commander Europe, the alliance’s top military commander, around 40,000 high readiness servicemen and women. It was tripled in size in the wake of Russia’s first invasion of Ukraine in 2014 when the Kremlin annexed Crimea and backed separatists in the east.

Under the anticipated new framework, the size of the capability will grow even larger and the name is expected to change to the Allied Reaction Force (ARF).

Each member state, with the exception of Iceland because it does not have an army, is understood to be looking at placing more of its own forces on a greater state of readiness.

They might be tasked with a specific area of NATO territory to defend under so-called “regional plans” that are still being worked on and are not expected to be finalised until the end of the year.

“The ARF will be about six times bigger than the NRF – significantly more,” said one military officer with knowledge of the thinking. This would mean some 240,000 military personnel.

A second military officer said he too had heard the six-fold figure.

A NATO source, however, said there has not been a decision yet and the final figure for the increase in high readiness forces would likely be lower.

Hardening of defences

Also part of NATO’s new plan is a further hardening of its defences in eight countries along its eastern and southeastern borders.

Mr Stoltenberg, in a public conversation with the Politico news site, revealed on Tuesday that allies will agree “to strengthen battlegroups in the east up to brigade level”. A battlegroup comprises about 1,000 troops, while a brigade can be between 3,000 and 5,000.

The comments point to a set of eight existing missions in the three Baltic states as well as Poland, Romania, Slovakia, Hungary and Romania growing further.

Allies, including the UK, the US, Canada and Germany are already contributing forces to the battlegroups there.

Posting troops overseas is expensive and, some allies argue, unnecessary to a point given NATO will likely have months of prior warning before Russia launched an invasion, which is what happened in the year-long build up to the Ukraine assault.

The UK, which heads the NATO mission in Estonia, grew its in-country presence to two battlegroups this year.

Ben Wallace, the defence secretary, has said it is “highly likely” a third will be added, turning the force into a small brigade of some 3,000 personnel, but some of the new troops could be stationed at home.

Germany, which leads the battlegroup in Lithuania, is also expanding its commitment to a brigade but has said it will have “preassigned” troops on standby at home in case needed.

By contrast, the Baltic states, with Russia in their backyard, want more boots physically on the ground.

Follow the Daily podcast on Apple Podcasts,  Google Podcasts,  Spotify, Spreaker

They would also like NATO to strengthen its defences even further, such as with an Israel-style “iron dome” air defence, able to blast Russian missiles out of the sky.

It is unlikely that such a move will be agreed in the near term, but enhanced air defences are being looked at, according to military and diplomatic sources.

Away from the immediate crisis, allies in Madrid will also unveil what is called a new “strategic concept” – effectively a blueprint for NATO on the threats and challenges it faces. This will include China, climate change, cyber warfare and evolving space capabilities.