The Concordat was NOT signed by Pope Pius XII! Nor was the German official who signed it a Nazi! Franz Joseph Hermann Michael Maria von Papen zu Köningen (29 October 1879 – 2 May 1969) was a German nobleman, Roman Catholic monarchist politician, General Staff officer, and diplomat, who served as Chancellor of Germany in 1932 and as Vice-Chancellor under Adolf Hitler in 1933–1934. A member of the Catholic Centre Party until 1932, he was one of the most influential members of the Camarilla of President Paul von Hindenburg in the late Weimar Republic. It was largely Papen, who believed that Hitler could be controlled once he was in the government, who persuaded Hindenburg to put aside his scruples and approve Hitler as Chancellor in a cabinet not under Nazi Party domination. However, Papen and his allies were quickly marginalized by Hitler and he left the government after the Night of the Long Knives, during which some of his confidants were "purged" by the Nazis.
Cardinal Pacelli was not a Nazi, and was working with several German states to secure the rights of Catholics and the Catholic Church in these states. While it is true that LATER Cardinal Pacelli became Pope Pius XII, he was not pope at the time and was working under Pope Pius XI.
The Reichskonkordat, signed on 20 July 1933, between Germany and the Holy See, while thus a part of an overall Vatican policy, was controversial from its beginning. It remains the most important of Pacelli's concordats. It is debated, not because of its content, which is still valid today, but because of its timing. A national concordat with Germany was one of Pacelli's main objectives as secretary of state, because he had hoped to strengthen the legal position of the Church. Pacelli, who knew German conditions well, emphasized in particular protection for Catholic associations (§31), freedom for education and Catholic schools, and freedom for publications. When negotiations for the concordat began, the Nazi Party had not yet taken over all political control of Germany.
Adolf Hitler was appointed Chancellor on 30 January 1933 and sought to gain international respectability and to remove internal opposition by representatives of the Church and the Catholic Centre Party. He sent his vice chancellor Franz von Papen, a Catholic nobleman and member of the Centre Party, to Rome to offer negotiations about a Reichskonkordat. On behalf of Pacelli, Prelate Ludwig Kaas, the outgoing chairman of the Centre Party, negotiated first drafts of the terms with Papen. The concordat was finally signed, by Pacelli for the Vatican and von Papen for Germany, on 20 July and ratified on 10 September 1933.
Between 1933 and 1939, Pacelli issued 55 protests of violations of the Reichskonkordat. Most notably, early in 1937, Pacelli asked several German cardinals, including Cardinal Michael von Faulhaber to help him write a protest of Nazi violations of the Reichskonkordat; this was to become Pius XI's 1937 encyclical Mit Brennender Sorge. The encyclical was written in German and not the usual Latin of official Roman Catholic Church documents. Secretly distributed by an army of motorcyclists and read from every German Catholic Church pulpit on Palm Sunday, it condemned the paganism of the National Socialism ideology. Pope Pius XI credited its creation and writing to Pacelli. It was the first official denunciation of Nazism made by any major organization and resulted in persecution of the Church by the infuriated Nazis who closed all the participating presses and "took numerous vindictive measures against the Church, including staging a long series of immorality trials of the Catholic clergy."
Rabbi David Dalin's The Myth of Hitler's Pope argues that critics of Pius are liberal Catholics and ex-Catholics who "exploit the tragedy of the Jewish people during the Holocaust to foster their own political agenda of forcing changes on the Catholic Church today" and that Pius XII was actually responsible for saving the lives of many thousands of Jews.
http://en.wikipedia.org/wiki/Reichskonkordat (wiki article)
http://en.wikipedia.org/wiki/Pope_Pius_XII (I know, it's a wiki, but very well documented)