Just to touch on the demographics; USA saw such a massive influx of immigrants because Europe was at its maximum capacity already. The demographics were already changing due to natural development as well. This seems quite uniform across most of Europe, the last big population booms happening right after WW2. Same changes seem to have happened in USA as well for that matter.
WWI and WWII's arguably largest catalyst was France's fear over losing her status as a great power because of her lower birthrate. France had always been the most populous nation in Europe since nations started existing, then Russia took over, then united Germany, then Austria-Hungary, UK and eventually even little Italy during the interwar period. Of course WWII considerably closed that gap. It's still worth noting that despite France's immigrants solving the demographic crisis, Germany has about 10 million more people on about 60,000 square miles less than the Germany the French a century ago were anxious about. Italy and the UK however were surpassed by the French due to immigration.