The Russian invasion of Ukraine in 2022 dramatically reshaped the European
security landscape. In Finland, public opinion on NATO had long been polarized
along the left-right partisan axis, but the invasion led to a rapid convergence
of the opinion toward joining NATO. We investigate whether and how this
depolarization took place among polarized actors on Finnish Twitter. By
analyzing retweeting patterns, we find three separated user groups before the
invasion: a pro-NATO, a left-wing anti-NATO, and a conspiracy-charged anti-NATO
group. After the invasion, the left-wing anti-NATO group members broke out of
their retweeting bubble and connected with the pro-NATO group despite their
difference in partisanship, while the conspiracy-charged anti-NATO group mostly
remained a separate cluster. Our content analysis reveals that the left-wing
anti-NATO group and the pro-NATO group were bridged by a shared condemnation of
Russia's actions and shared democratic norms, while the other anti-NATO group,
mainly built around conspiracy theories and disinformation, consistently
demonstrated a clear anti-NATO attitude. We show that an external threat can
bridge partisan divides in issues linked to the threat, but bubbles upheld by
conspiracy theories and disinformation may persist even under dramatic external
threats