NATO

Aside from the check on Russian aggression, NATO must be understood in the light of the historical belief - held and expressed by consecutive Presidents - that only the presence of the US prevents Europe from slipping into inevitable warfare on the continent.

Indeed, the 73 years of peace post WWII is double the length of any prior peace. HW Bush was adamant a reunited Germany stay in the alliance just for that reason. 

But is that a true concern in the era of the EU? And if it still is, is it a good enough reason for our commitment? Would we inevitably be drawn into another European squabble if we left, or could we sit it out (if Russia kept to their own borders)? 

I don't know.