After Russia's invasion of Ukraine, Sweden changed its historic position of non-alignment and applied for membership in the NATO military alliance. This decision coincided with a shift in public opinion in favor of NATO membership, in what has been described as the largest and fastest shift in public opinion that has ever been measured in Swedish history. We examine two aspects of this “rally around the flag effect” to explain this change, the effect of the crisis event itself and the effect of political elite influence by analyzing within-individual changes of a Swedish university student population at three different time points. We find that after the Russian invasion but before the decision was made to join NATO, public opinion was still split with roughly 40% still opposed. However, after the decision was made to apply for membership to the alliance many people changed their position (32%), either making up their minds or switching position entirely. We show that these changes were likely in response to the decision to join, and that they were moderated by individuals’ previously established political preferences.