Kyiv’s counteroffensive in the north-east of Ukraine appeared to take everyone by surprise, not least Russia’s war planners who had been moving troops south to meet an offensive in the Kherson region which Ukraine had been trumpeting about for several weeks. Ukraine’s president, Volodymyr Zelensky, is claiming that his military has won back 700 square miles of territory, including key Russian supply bases Kupiansk and Izium.

Ukraine’s military success must have equally surprised politicians and pundits around the world who have, over the last six months, urged Ukraine to offer concessions in order to secure a peace settlement with Russia. Giving up territory in the east or pledging to remain neutral would save Ukrainian lives and reduce the risk of a Russian nuclear strike, they argue. But this has raised the question as to what sort of settlement would be acceptable to Ukrainians and whether they would support ceding territory or sovereignty to end the violence.

Ukraine has a just cause for war – self-defence. Russian opinions excepted, this is something most of the rest of the world agrees on. But even a war with a just cause may not be worth fighting. Moral philosophers and lawyers caution that a war of self-defence must still be proportionate – the projected costs should not exceed the benefits.

Calls on Ukraine to negotiate or surrender often echo this argument. Ukraine can’t expect to defeat its large neighbour in the long run, so it should give up self-defence now to limit the costs of the war. But should resistance to aggression really be constrained by such cost-benefit calculations?