EU defence ministers assembled in Brussels on September 28 to try to formulate a response to increasing Russian aggression in Europe. The talks focused on Russia’s strategy of hybrid warfare – that is, hostile acts that fall below the threshold of actual war.

These tend to include sabotage, disinformation, airspace violations, political interference and cyberwarfare. One part of Europe that has become familiar with these kinds of actions in recent years are the Baltic states – Estonia, Latvia and Lithuania. Quite apart from being nervous because of their physical proximity to Russia, these countries have had to deal with regular acts of hybrid warfare for some years now.

These tactics fall into what is often called the “grey zone” – they generally fall shy of the criteria needed to invoke Nato’s Article 5 clause of mutual self defence, which considers an attack on one member state as an attack on all. The increased use of hybrid actions has intensified in recent years, and the question of how to react to these grey zone threats has become a matter for debate inside Nato. The agreement is that a “game-changing” cyberattack could theoretically merit such a move.

One of the difficulties is that these grey zone tactics come with a degree of plausible deniability. We saw this in 2014, when Russian troops entered parts of Ukraine, including Crimea, with no insignia (sometimes referred to as the “little green men”).

Russia has also claimed that its aircraft and drones violating other countries’ airspaces have done so accidentally. And it’s often hard to........

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