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."
I suspect that your text or your teacher has a set answer they want you to reproduce as this is a question that could have a large variety of answers. I would encourage you to look for that in your book or your notes.
There are so many ways that Europe's physical environment has affected it over the years. Here are a few examples:
- Europe is broken up by large mountain ranges in various places. This has allowed many different cultures to arise and to persist even though the land area of Western Europe in particular is not large.
- The physical environment of Greece led to the creation of city-states. This, in turn, led to the creation of democracy in Athens. Athenian democracy has had a huge impact on the history of Europe.
- Some scholars have argued that the relatively hostile environment of Europe helped cause Europeans to achieve world-wide supremacy. This argument holds that Europeans were forced to become innovative and technologically sophisticated by the harsh environment. This gave them an advantage over people in more pleasant climates who did not have to work as hard to survive. This theory is controversial, however.
Once again, please consult your text and/or notes in case there is an answer you are supposed to get from those sources.