The concordat was an interstate agreement that regulated the affairs of the church in Germany. It did not give any support to the Nazi agenda. The UK and Germany signed the Anglo-German Naval Agreement in 1935. Such treaties do not imply support for Nazi regime but were agreements with what was then the lawful government of Germany. There was nothing nefarious about the concordat despite your wishes to the contrary.
You wrote:
“The US never signed a Reichskonkordat with Hitler. The Roman Catholic church did...”
What is that supposed to mean? The Church wanted to protect Catholics in Germany. If there were 35 million Americans living in Nazi Germany you can bet we would have signed a treaty with the Nazis to protect them. We already were selling them war supplies throughout the 1930s! This is from a lefty union, but they get the facts right: http://www.ranknfile-ue.org/uen_nastybiz.html
Again, if there were 35 million Americans living in Nazi Germany you can bet we would have signed a treaty with the Nazis to protect them.
The Vatican signed a concordat with Hitler , thinking that Hitler, like Mussolini, would keep that agreement out of self interest. The immediate concern was to prevent another Kulturkampf. It didn’t work. Mussolini was a gangster, Hitler and his gang was antichrist. Thy had no intention of allowing the Church to educate the youth of the country. They could not be appeased. That didn’t keep the pope and most of Europe from thinking they could be.