This week's Nato summit in Madrid comes at a critical time in the alliance's 73-year history. Russia's invasion of Ukraine has been described as the biggest strategic shock to the West since the 9/11 attacks of 2001. Nato is the sole military alliance capable of defending Europe from further Russian aggression, but does it have a strategy?
Less than three years ago, France's President Macron declared Nato to be "brain dead". Yet from the moment Russian tanks rolled across the border into Ukraine, the Western response has been remarkable for its unity, speed and vigour. It has been reinvigorated with renewed purpose - strengthening borders and supplying weapons.
On the eve of the summit in Madrid, Nato Secretary General Jens Stoltenberg announced what he called "a fundamental shift in the alliance's deterrence and defence", strengthening its defences on its eastern borders and raising its rapid response force to more than 300,000 troops.
The alliance faces numerous challenges, from hybrid warfare to the destabilisation of the Balkans to cyber attacks, the militarisation of space and what to do about China's growing military power. For the first time, this Nato summit will address what it calls "the challenges that Beijing poses to our security, interests and values", Mr Stoltenberg said. Here are some of the most pressing issues likely to be on the table this week.
Nato faces a balancing act. The world's most powerful military alliance, comprising 30 member states, three of them with nuclear weapons (US, UK and France), does not want to go to war with Russia. President Putin has repeatedly reminded the West he has a massive nuclear arsenal and even a low-level cross-border clash could quickly escalate out of control.
Early Western inhibitions about not upsetting Moscow by sending heavy weapons to Kyiv have been swept aside as gruesome details of alleged Russian war crimes and atrocities have emerged, backed up by satellite data.
The Madrid summit will need to set out the extent of military help Nato countries can provide and for how much longer.
For now, Moscow is winning in the Donbas, the largely Russian-speaking region of eastern Ukraine, albeit at massive cost in lives and material damage. Expectations are Russia will try to hang on to these territorial gains, perhaps annexing them in the same way it did with Crimea in 2014.
In the absence of a peace treaty, Nato will face a new dilemma later. Does it continue to arm the Ukrainians as they try to claw back land Moscow now considers to be legally part of the Russian Federation? The Kremlin has indicated Western weapons striking Russian soil crosses a red line so the risks here of escalation would dramatically increase.