Why is it that Jewish people are looked at as a nationality (as they were during WWII) rather than as members of a religion?

Jews have typically been seen as members of a nation (or race or ethnic group) rather than as members of a religion because, at least in Europe, most Jews tended to share more or less the same “racial” background.  In addition, these Jews were typically segregated and discriminated against, making it easy to identify them and drastically reducing the number of people who converted to or from Judaism.

The people that the Western world knows...

Jews have typically been seen as members of a nation (or race or ethnic group) rather than as members of a religion because, at least in Europe, most Jews tended to share more or less the same “racial” background.  In addition, these Jews were typically segregated and discriminated against, making it easy to identify them and drastically reducing the number of people who converted to or from Judaism.

The people that the Western world knows as Jews are largely Ashkenazic Jews.  These are Jews who moved into Central and Eastern Europe during the Middle Ages. This group of Jews became the dominant group in all of European Jewry.  Because of this, most European Jews (and later, American Jews as Ashkenazim moved to the United States) had a shared ancestry.

Moreover, this group’s “blood” was not very diluted by intermarriage with other groups. Throughout history, Jews in Europe have been segregated and have often been persecuted.  Up until very recently, this meant that Jews married other Jews almost exclusively.  Because of this, Jews in Europe shared some physical characteristics much more than Lutherans or Catholics, for example, did.  What this meant was that people could identify Jews by sight where they could not identify people of other religions in the same way.

Because of the segregation and persecution, there was also very little in the way of conversion to and from Judaism.  Very few Europeans would willingly have become Jewish and faced possible discrimination and persecution.  Most Jews either wanted to keep their heritage or knew that they would be unable to “pass” as Christians.  This contributed to the situation in which it was easy to tell who was and was not a Jew.

Because Jews often had distinguishing physical characteristics, and because they tended to be segregated, it was easy to see them as members of a different “race.”  Therefore, people came to feel that the religion and the “race” were inextricably linked.  They did not feel that a Jew was simply a person who believed in Judaism.  Instead, a Jew was a person of a certain “racial” background, one that could not be thrown off simply by espousing different beliefs.