You cannot enter a foreign Country with a one-way plane ticket. You must either have a round trip ticket or show proof of leaving their country and going to another (if not returning to your home country). Customs can, and probably will, put you back on the airline that you flew with and send you all the way back home. Traveling within Europe is easier once you arrive from the USA, but they will be very picky on your first hop from the US to their country. So, plan ahead and purchase the right tickets.