Kook led the Irgun's efforts in the United States during World War II and the Holocaust in order to promote Zionism, attempting thereby to save the abandoned Jews of Europe.

[1] He also attended classes in Jewish Studies at the Hebrew University, where he became a member of Sohba ("Comradeship"), a group of students who would later become prominent in the Revisionist movement, including David Raziel and Avraham Stern.

At the founders' request, Kook traveled to the United States with Jabotinsky in 1940,[2] where he soon served as the head of the Irgun and revisionist mission in America, following the elder's death in August.

The Bergson Group was composed of a hard-core cadre of ten Irgun activists from Europe, America and Palestine, including Aryeh Ben-Eliezer, Yitzhak Ben-Ami, Alexander Rafaeli, Shmuel Merlin, and Eri Jabotinsky.

As information about the Holocaust began to reach the United States, Kook and his fellow activists became more involved in trying to raise awareness about the fate of the Jews in Europe.

On March 9, 1943, the Group produced a huge pageant in Madison Square Garden written by Ben Hecht, titled "We Will Never Die", memorializing the 2,000,000 European Jews who had already been murdered.

Forty thousand people saw the pageant that first night, and it went on to play in five other major cities including Washington, D.C., where First Lady Eleanor Roosevelt, six Supreme Court Justices, and some 300 senators and congressmen watched it.

Some of the members of the Emergency Committee to Save the Jewish People of Europe were: Hillel Kook (Peter Bergson) and Alex Hadani Rafaeli, Alex Wilf, Arieh Ben-Eliezer, Arthur Szyk, Ben Hecht, Ben Rabinowitz (Robbins), Eri Jabotinsky, Esther Untermeyer, Gabe Wechsler, Senator Guy Gillette, Harry Selden, Johan Smertenko, Konrad Bercovici, M. Berchin, Shmuel Merlin, Sigrid Undset, Stella Adler, Congressman Will Rogers, Jr., Yitzchak Ben-Ami, and John Henry Patterson.

There were many others who actively supported the Bergson Group, including a number of the best known people in Broadway theatre and Hollywood, probably due to Hecht's contacts (such as Kurt Weill).

At the time Kook established the Hebrew Embassy in Washington and was in the habit of saying "Palestine Free State", which Begin thought left too much potential for bi-nationalism.

In July of 1943, Kook and his followers began a campaign to lobby Congress for a resolution to create an independent agency to save European Jews.

[8] Among those trying to stop the Bergson Group's rescue activities were Chairman of the Foreign Affairs Committee Congressman Sol Bloom, and World Jewish Congress leaders Stephen Samuel Wise and Nahum Goldmann.

[9] One of the Committee's more memorable activities was a protest in Washington, D.C. that Kook organized, which took place on October 6, 1943—three days before Yom Kippur.

The march garnered much media attention, much of it focused on what was seen as the cold and insulting dismissal of many important community leaders, as well as the people in Europe they were fighting for.

That night, Begin spoke on the radio and commanded his followers not to take revenge and prevented a probable civil war among Israel's Jewish residents.

Kook, who had returned to Israel after a ten-year absence, was now confronted with the reality that the country and movement he had fought for bore little resemblance to his ideals.

Profoundly disillusioned with the Israeli political process and future of the Revisionist movement, Kook left Israel in 1951 with his wife and daughter.

[12][13] Like Ratosh, Kook was influenced by Adolf Gurevich, a Betar activist with connections to Bergson Group members Shmuel Merlin and Eri Jabotinsky.

He suggested amending the Law of Return for Jews residing outside Israel to be limited to a few years after Independence (1948) and to consider prospective immigrants on an individual, and not on a national or religious basis, except for cases of immediate danger.

Since the late 1990s, some historians have attempted to re-examine and evaluate the significance of his activities during World War II and his role as a political opponent of Begin.

David Wyman and Rafael Medoff, co-authors of a 2002 Kook biography,[15] suggested that, despite the frequent obstruction by the modern mainstream American Jewish and Zionist establishment, Kook's rescue group's activism was the major factor in establishment of the War Refugee Board and that it was an instrument rescuing approximately 200,000 (probably an over-estimate), partly by means of the Raoul Wallenberg mission.

The above main obstructors of the rescue committee Kook formed and led received much recognition and very high positions after the war in the mainstream Jewish and Zionist world.

Film maker Pierre Sauvage directed a documentary about the activities of Kook during World War II: Not Idly By - Peter Bergson, America and the Holocaust.

The more recent 2009 Against the Tide, directed by Richard Trank and produced by Moriah Films of the Simon Wiesenthal Center, includes narration by Dustin Hoffman.

We responded as a human and as a Jew should.I, who was the liaison officer of the Irgun central command with Jabotinsky, and who accompanied him almost daily for four years—remained loyal to his teachings.

But I am also certain that had Jabotinsky lived today, he would have argued that now, after we've achieved our independence, our mission is to attain peace in order to establish the Israeli people as the political heir of the Jewish people.There is no exile.