There are a couple of things that could upset the geopolitical status quo in a way that could lead to war in Europe. A massive influx of refugees from the Midd

There are a couple of things that could upset the geopolitical status quo in a way that could lead to war in Europe. A massive influx of refugees from the Middle East in particular could have the effect of destabilizing any of the countries that accept them. It could also embolden and empower nationalists who in turn may make extreme choices that could lead to open hostility. If there's a resurgence in radical Climate change may indeed be the cause of large refugee crises hitting Europe. Depending on how bad climate change related events are, it could degrade the ability of governments to maintain order and provide public services. Those are things that can lead to unpredictable and violent outcomes. Then there's the specter of Putin's Russia using covert and direct military action to reclaim former Soviet territory, subvert European governments, spread nationalist propaganda, etc. And there's always the Black swan event that no one had on their radar that could massively change the playing board. We've already seen a big upheaval with Brexit, the Trump presidency, and the Syrua refugee crisis. More of that kind of stuff would be really bad news for Europe.