During the Cold War, when Western Europe designated the countries that are part of the NATO treaty and under American influence, the term was often used as a counterbalance to Eastern Europe that was under Soviet influence. The borders between Western and Eastern countries were securely defended, especially on the Eastern side. These borders were also called the Iron Curtain.
Until recently, Western Europe could safely be said to comprise the countries of the European Union plus Iceland, Switzerland, Liechtenstein, Andorra, Norway, San Marino, and Monaco. However, many countries that are (or were) located in Eastern or Central Europe, such as Poland and the Czech Republic, are now seeking inclusion into the European Union. It is likely that the increased economic and cultural ties with Western European countries will lead to a westward migration as inclusion occurs. This will lead to yet another change of the definition of Western Europe in the future.