These costs could be incurred by making the aggressor’s objective difficult to achieve: in essence, defence, much as Ukraine is doing to Russia at present. The costs could be economic, as we’ve seen from the EU and international community imposing some of the most devastating economic sanctions in history on Russia. Or they could be punitive, such as violence against the interests of the aggressor, potentially involving nuclear weapons.
But it is the threat of these costs that deters, not their imposition. If the threat is not credible, or if the deterrer does not have the capability to impose the costs, the risk might be perceived as low. It is clear that the Russian leadership misjudged the ability of the Ukrainian forces to defend their country, and were therefore not deterred by the prospect of unacceptable costs in achieving their objective (whatever that actually is).
Despite the indications of impending invasion, the international community failed to deter Russian aggression. Prior to the invasion, there was no coherent message from the international community about the severity and scale of sanctions that Russia would face. Indeed, the only coherent message seems to have been that there would be no military intervention. Had the EU, USA and NATO declared in the weeks before the Russian invasion that they had agreed they would impose debilitating economic sanctions, the Russian leadership would have been faced with a very different risk calculus to that it faced on 24 February.
Now, with Russian forces already committed to the attack on Ukraine, devastating sanctions have been imposed and the costs they have incurred appear far more debilitating than Russian leadership anticipated, and might well now outweigh the gains anticipated from the invasion. But now the Russian leadership is committed to this course of action and is faced with two options: a humiliating climbdown; or escalation of violence in an effort to compel Ukraine to surrender.
The lesson for the international community is that deterrence works only if the threats made are coherent and credible, and made before the aggressor commits to a course of action. In the midst of his stalled invasion which looks increasingly like it will become less discriminate and more violent, Putin knows this: an increase in the readiness level of Russian nuclear forces was announced on 27 February.
Putin is now seeking to use nuclear deterrence against Europe and NATO to influence their decisions about further engagement; what costs is the international community prepared to risk for Ukraine?