In January 2006, a book (in Dutch) appeared in The Netherlands, written by Sybe Rispens, entitled  Einstein in the Netherlands. One chapter of this book discusses the relationship between Albert Einstein and Debye. Rispens discovered documents that, as he believed, were new and proved that, during his directorship of the Kaiser Wilhelm Society, Debye was actively involved in cleansing German science institutions of Jewish and other "non-Aryan elements". Rispens records that on December 9, 1938, Debye wrote in his capacity as chairman of the Deutsche Physikalische Gesellschaft (DPG) to all the members of the DPG:
In light of the current situation, membership by German Jews as stipulated by the Nuremberg laws, of the Deutsche Physikalische Gesellschaft cannot be continued. According to the wishes of the board, I ask of all members to whom these definitions apply to report to me their resignation. Heil Hitler!
Many biographies published before Rispens work, state that Debye moved to the US because he refused to accept German citizenship forced on to him by the Nazis. He planned his departure from Germany during a visit with his mother in Maastricht in late 1939, boarded a ship in Genoa in January 1940 and arrived in New York in early February 1940. He immediately sought a permanent position in the US and accepted such an offer from Cornell in June 1940. That month, he crossed the US border into Canada and returned within days on an immigration visa. He was able to get his wife out of Germany and to the US by December 1940. Although his son already was in the US before he departed, Peter Debyes 19-year-old daughter and his sister-in-law did not leave. They lived in his official residence in Berlin and were supported by Debye's official Berlin wages (he carefully maintained an official leave of absence for this purpose).
Further, Rispens alleges that Albert Einstein in the first half of 1940 tried to prevent Debye from being appointed in the United States at Cornell. Einstein allegedly wrote to his American colleagues: "I know from a reliable source that Peter Debye is still in close contact with the German (Nazi) leaders" and, according to Rispens, called upon his colleagues to do "what they consider their duty as American citizens". To support this, Rispens refers to a well-known letter from Debye to Einstein and Einstein's response to it. Van Ginkel investigated 1940 FBI reports on this matter and traced the "reliable source" to a single letter directed to Einstein and written by someone whose name is lost. This person was not known personally to Einstein and, according to Einstein, probably did not know Debye personally either. Moreover, this accusatory letter did not reach Einstein directly but was intercepted by British censors who showed it to Einstein. Einstein sent the British agent with the letter to Cornell, and the Cornell authorities told Debye about the affair. Thereupon Debye wrote his well-known 1940 letter to Einstein to which Einstein answered. The latter two letters can be found in the published Einstein correspondence.
Rispens alleges that Debye sent a telegram to Berlin on 23 June 1941 informing his previous employers that he was able and willing to resume his responsibilities at the Kaiser Wilhelm Institut, presumably to maintain his leave of absence and keep the Berlin house and wages available for his daughter. A copy of this telegram has not been recovered thus far. In summer 1941, Debye filed his intent to become a US citizen and was quickly recruited in the US to participate in Allied War research.
It has been well documented in many biographies, and also in Rispens' book, that Debye and Dutch colleagues helped his Jewish colleague Lise Meitner in 1938–1939 (at great risk to himself and his family) cross the Dutch-German border to escape Nazi persecution and eventually obtain a position in Sweden.
Predating Rispens work, and in contrast to it, an article by Rechenberg appeared 18 years earlier concerning Debyes letter. The article describes Debye's missive in more detail and presents a very favorable picture of Debye in his efforts to resist Nazi activists. Moreover, this article points out that Max von Laue, well known for his anti-Nazi views, gave his approval to the letter from the DPG chairman.