Sanna Marin’s time as Finland’s Prime Minister may be coming to an end, but the 37-year-old leader has presided over a pivotal moment in her country’s history. On Tuesday, Finland joined NATO, doubling the alliance’s land border with Russia. It’s the precise opposite of what Vladimir Putin wanted to happen when he invaded Ukraine last year, and a tectonic shift in the balance of power in the Baltic Sea. Russia’s strategy since Napoleonic times has been to menace Finland and stop it from joining hostile military alliances. In a matter of months, centuries of careful Russian foreign policy and diplomacy have come crashing down.