AbstractFinland's decision to apply for membership in NATO is a case of bottom‐up driven foreign policy. Public opinion had been rather stable for more than 25 years in the NATO issue with less than 30 per cent of the people supporting Finland's membership in NATO but almost overnight during the week when Russia launched its invasion of Ukraine, a majority of Finns were in favour of it. Only after that shift in public opinion did the decision‐makers support the idea of joining NATO. How was the bottom‐up driven change possible? First, the resistance to NATO membership in Finland was common but not deep. Second, in the run‐up to the war, Russia had demanded that NATO put a stop to its open door policy not only with regard to Ukraine but also for Finland. Third, Russia's unprovoked war on Ukraine constituted a particular frame in Finland, because the 1939 Winter War analogy was a key part of the collective memory. Finally, due to a strong tradition of national consensus in foreign and security policy, the shift in the public opinion became overwhelming: when the tide had turned, only very few resisted. In sum, four factors enabled the public opinion change: a rational basis for opinion change, a sense of urgency, an emotional push and the consensus effect. Although public opinion is often seen as stabilising and retardant force particularly in foreign and security policy, it can also be a driver for change when suitable conditions such as those in Finland exist.