Brexit in the UK causes Italy or France to have a similar referendum.
Le Pen wins an election in France. Europe now has a fractured EU.
The EU, for all its many awful faults, has prevented a war in Europe for longer than ever before.
The EU is also a major force in suppressing Putin’s military ambitions. European sanctions on Russia really hit the economy, and helped temper Russia’s attacks on Ukraine (there is a reason bad guys always want a weaker European Union).
Trump wins in the US. Trump becomes isolationist, which weakens NATO.
He has already said he would not automatically honourNATO commitments in the face of a Russian attack on the Baltics.