ENhis article analyses changes in national defence policies across theBaltic Sea region after the exogenous shock of the Ukraine crisis in 2014. It draws on Punctuated Equilibrium Theory for theoretical guidance, particularly to understand the mechanisms leading tolarge-scale policy change after periods of relative stability. Sincethe outbreak of the Ukraine crisis, defence budgets in the Baltic Sea region have increased dramatically. Baltic Sea states investedin neglected capability development and redirected policyplanning towards territorial and collective defence. By contrast, the Russian-Georgian conflict in 2008 had a negligible effect on the paradigm guiding defence planning processes, delayed andlukewarm policy responses let adaptation pressure grow. The empirical findings suggest that policy change reinforced post-2014 because a normative consensus emerged on why Europe’s security order is at risk, a critical threshold of urgency was reached, setting off positive feedback cycles, and receptive policy venues, such as NATO, as well as capable external policy actors, such as the USAand the UK, put Baltic Sea security back on the agenda. Keywords: Baltic Sea; European security; defence cooperation; Ukraine; NATO; exogenous shock; policy change; Punctuated Equilibrium Theory. [From the publication]