How has Europe's physical environment impacted Europe?

Geographical entities like mountains and rivers, in
addition to being geographical boundaries, are frequently used as political
boundaries. The many major rivers throughout Western Europe also unquestionably assisted
in the development of trade.  As development of commerce within Europe progressed, trade
expanded further and further afield until it was trading with the rest of the world.
Conversely, mountainous regions isolated areas and restricted trade. Switzerland, for
example, is ringed with mountains and became its own political entity because it was
geographically contiguous.  The mountains, although restricting trade in times past,
were also useful for defense, and accounts for the famous "Swiss
Neutrality."