Why was anti-Semitism widespread in Europe before the Nazis?
The Nazis did not invent anti-semitism. They just used it to gain and maintain power. Anti-semitism is thousands of years old, and most likely stems from the refusal of Jewish people to change their way of life and religion and because once they were dispersed from their own country, Israel, they were "foreigners," no matter what countries they lived in. Spain, in the 14th century, killed Jewish people who refused to convert to Catholicism. England, during certain periods of its history, did not permit Jews to live there. Other European countries had policies whereby Jewish people could not vote, own property, etc. Most "isms" stem from a people's "otherness," and since Jewish people were different, anti-semitism was a consequence early in their history.
Jews were persecuted throughout Europe for centuries before the Nazis turned their sights on them. During the Roman Empire, Jews were fed to lions at the Colosseum for entertainment. Later, Jews were blamed for blood libels and spreading the plague. In the fifteenth century, Jews were given the choice of converting to Christianity or facing death. Pogroms and massacres took place in eastern Europe in the ninteenth century, killing hundreds of thousands of Jews.
Still reeling from the Treaty of Versailles which required Germany to pay reparations and concede land, the Nazis were eager to find a scapegoat for Germany's post-war decline.
Christian anti-semitism stems from the death of Jesus in around 33A.D.. Jesus was a Jew. There is nothing in the bible that says he was trying to set up a new religion. He calls The Jewish Temple in Jerusalem, 'My Father's House'. He was a Jew and his religion was Judaism. Albeit he was a radical Jew who was trying to alter his native religion, but Jesus was not a Christian.
(Now, remember that at the time of Jesus, The Holy Land was occupied by the Romans and the mighty Roman empire was comparitively more powerful than America is today.)
Jesus was killed by the Romans. They crucified him. They nailed him up. And after his death, his message was doctored by Paul (Jesus's only Roman follower) and taken to Rome and, eventually, the Romans accepted Christianity. (whereas, in the Holy land, Jesus's message quickly died out.)
So, the problem for the Romans was this... If the Roman Empire became Christian, how could they defend the fact that they had killed The Son of God. How could the killers of Christ be the promoters of Christ? ... So they blamed The Jews instead! Roman Emperor Constantine changed The Bible so that it said The Jews "asked" the Romans to kill Christ on their behalf. Then they spent the next 2000 persecuting Jewish people as 'christ killers'.
European anti-semitism stems from The Roman Catholic Church's absolute determination to not accept responsibiliy for the fact that the Roman Authorities killed Jesus by the 'signature' Roman punishment of cruxifiction. The Romans killed a Jew called Jeshua Ben Joseph (Jesus) and then, later, they became The Roman Catholic Church. And they have aggresively shifted the blame for the death of Christ ever since.
'I wash my hands of all responsibility' said Pontius Pilate... hmmm, how very convenient. But he still sent him off to be nailed to a cross.