1 Introduction

Despite a period of momentum building, the Russian invasion of Ukraine on Feb 24, 2022 came as a shock to most observers. The shock was most acute in Ukraine but was felt also in countries bordering Russia. Finland, the militarily non-aligned European country that shares a 1344-kilometer border with Russia, witnessed a sharp shift in its public opinion on NATO membership, based on a reappraisal of the external threat posed by Russia. Traditionally, around 20 percent of the Finnish population had been in favor of joining NATO [1]. Russia’s invasion of Crimea in 2014 increased the number to 25–30 [1], but after the invasion of Ukraine in 2022, support for joining NATO soared as high as 70–80 percent [2].

Behind this major change in opinion, the rising external threat seems to have had a depolarizing effect on the Finnish NATO discussion. For long, Finnish opinions on NATO embodied a polarization that was largely partisanship-based: voters of the main right-wing party (National Coalition) were largely in favor of joining, whereas voters of left-wing parties were the most vocal opponents of NATO [3]. After the invasion, however, many left-wing supporters changed their opinion, and eventually the Finnish parliament almost unanimously voted in favor of joining NATO (188 for, 8 against).

Social media opens an unobtrusive observation window [4] into whether and how this depolarization took place among the more politically active and partisan segment of the population [5, 6], including political elites who often play an important role in steering the discussion [7, 8], as well as fringe communities that subscribe to conspiracy theories and disinformation [34] and diffusion networks [33]. Bessi et al. [35] showed that conspiracy news consumers are more focused on diffusing within-group content and interacting with within-group actors, which points to the potential stability of conspiracy-based polarization. Zollo et al. [50] further added that users within the conspiracy echo chamber rarely interact with debunking posts, and when they do, their interest in conspiracy content actually increases after the interaction. Echoing these findings, our results more concretely demonstrate the resilience of conspiracy-/disinformation-based polarization. We show that consumers of conspiracy theories and disinformation formed a separate retweeting bubble, and further, that they were reluctant to change their opinions or communication patterns even in the face of a dramatic external threat and otherwise bridged partisan divides. This alerts us to the fact that conspiracy theories and disinformation are consumed by polarized actors that are even more entrenched than partisan actors, and that it can be extremely difficult to pave a way toward conversation and consensus with them.