Why peace? Well, the EU originates from the European Coal and Steel Community, which was formed in the aftermath of World War II with the idea of joint control over the key materials that might be used in a future war, namely coal and steel, by six countries which had formerly been bitter enemies (such as Germany and France). This aim has achieved the longest period without wars in Western Europe in recorded history, a peace of over 65 years. (This excludes the Balkan Wars, which took place outside the then EU.)
It is common knowledge that the EU has continued to expand, with a number of European countries having recently joined and others on the way. The EU sees enlargement as an important way to ensure that peace and prosperity are spread to some of the poorer and historically more troubled parts of Europe, particularly the Western Balkans, which fought vicious wars as late as 1991–99 when the former country of Yugoslavia broke up. It is easy to forget that these wars, in countries only a few hours by plane from the UK, resulted in over 150,000 deaths and 3 million refugees, and that ethnic tensions still run high in some of these places. The hope is that these nations will settle any present and future differences peacefully through membership of the EU.