DW: From time to time, the Russians send out signals that they are willing to use nuclear weapons. Is this just propaganda, or is there a growing risk of these weapons being used strategically — or at least tactically — on the battlefield in Ukraine?

Anders Fogh Rasmussen: I don't consider these warnings real, so I am not concerned about the threats. It's an attempt to intimidate NATO allies into reducing weapons deliveries to Ukraine. Putin knows that were he to use weapons of mass destruction, whether tactical nuclear, biological, or chemical weapons, there would be a determined response. And Putin would lose. If we allow these threats from Moscow to impact us, then this brutal tyrant gets to have the last word in the conflicts he initiates.

How would NATO respond to such an attack?

President Joe Biden put it well after the last NATO summit when he said that if Putin uses nuclear, chemical, or biological weapons, he will cross a red line, and we will respond. But Biden wasn't clear. He didn't specify what kind of response that would be and precisely what sort of red line would need to be crossed. Such uncertainty also plays a role in maintaining credible deterrence.

To date, too many NATO leaders have been too eager to exclude this or that reaction to Russia's actions in Ukraine. We should never do that. If we want our deterrence to be credible, the adversary needs to be unsure about where the red line is. And he should also be uncertain about how and when we will respond if he crosses it. Other statements only serve to expand Putin's room for maneuver. 

Was it a mistake for Biden and NATO to declare that they would not send a single soldier to Ukraine? Should this issue have been left open as part of the "strategic ambiguity" you mention?

Honestly speaking, yes. But then, Biden did the right thing when he remained unclear about how exactly NATO would react after the last NATO summit. That should have been done much earlier, but I think NATO has learned its lesson. Because when you exclude troops on the ground, when you exclude a no-fly zone, when you rule out other actions, you tell Putin what he can do without any reaction from us.

NATO countries are spending a lot of time debating the types of weapons supplied to Ukraine, how not to escalate and how not to provoke. Where do you see the line between what can be delivered and what cannot? 

We should not take into account what Putin might feel is a provocation. Putin is the aggressor who has attacked a peaceful neighbor. And according to the UN Charter, every country has the right to self-defense. Moreover, the government of such a state has the right to ask for help from other countries in this self-defense. Whatever the Ukrainians have asked for, we should deliver if we can. 

The alliance is sometimes accused of provoking Moscow. In this respect, the pope used the phrase "barking at Russia's gates" in a recent interview.

I completely disagree with that. At no time have we threatened Russia. We have not provoked Russia with NATO enlargement. We did not campaign for NATO enlargement, but merely accepted applications from former communist states that wanted protection from Russia. So perhaps the Russians should ask themselves why all their neighbors are looking to NATO for security guarantees? We have reached out to Russia on several occasions throughout this whole process. The Russians had a sizeable permanent representation at NATO headquarters because we wanted to say to them, "What we are doing is not directed against you. You can watch with your own eyes what is going on." We had the NATO-Russia Council, which met once a month and provided a framework for cooperation, consultation and joint action preparations. In 2010, at the NATO summit in Lisbon, we decided to develop a strategic partnership between NATO and Russia. I think the pope should re-examine his history.

Wasn't it a mistake for NATO to declare that its door is open to Ukraine, but at the same time not to offer a path to membership?

Seen retrospectively, we have made a lot of mistakes. One of them was not granting Ukraine and Georgia our "Membership Action Plan" (MAP) at the NATO summit in Bucharest in 2008. I was in favor, the US was in favor, the UK was in favor, but Germany and France were against it, so we could not agree on it. This sent the wrong signal to Putin. And a few months after the Bucharest Summit in 2008, it was he who sent out his clear message — he attacked Georgia. Moreover, we reacted far too weakly when Russia occupied Crimea in 2014 and began its occupation of Donbas. 

Putin does not want a political and diplomatic solution for the time being. It is therefore of the utmost importance that we continue the delivery of heavy and sophisticated weaponry to the Ukrainians to fight the invaders, which hopefully will bring a clear defeat for the Russians.

Anders Fogh Rasmussen was Secretary-General of NATO from 2009 to 2014, immediately before Jens Stoltenberg, the current head of the alliance. Before that, Rasmussen was Danish prime minister for eight years. He currently runs the consultancy firm Rasmussen Global.

Edited by: Aingeal Flanagan and Bettina Marx