[2] During World War II he worked on Tube Alloys and the Manhattan Project, but left the Los Alamos Laboratory on grounds of conscience after it became clear to him in 1944 that Germany had ceased development of an atomic bomb.

[6] His father, Zygmunt Rotblat, built up and ran a nationwide horse-drawn carriage business, owned land and bred horses.

[6] He sat the entrance examinations of the Free University of Poland in January 1929, and passed the physics one with ease, but was less successful in writing a paper about the Commission of National Education, a subject about which he knew nothing.

[13] In early 1939, he envisaged that a large number of fissions could occur and if this happened within a sufficiently short time, then considerable amounts of energy could be released.

[14][15] In 1939, through Wertenstein's connections, Rotblat was invited to study in Paris and at the University of Liverpool under James Chadwick, winner of the 1932 Nobel Prize for discovering the neutron.

[18] Tola was trapped, and desperate efforts in the ensuing months to bring her out through Denmark (with the help of Niels Bohr), Belgium, and finally Italy came to nothing, as each country in turn was closed off by the war.

[22] In February 1944, Rotblat joined the Los Alamos Laboratory as part of Chadwick's British Mission to the Manhattan Project.

[22] Although he was upset by the morality of the project, he believed the allies needed to be able to threaten retaliation in case Germany developed the bomb.

In 1985, he related that, at a private dinner at the Chadwicks' house at Los Alamos in March 1944, he was shocked to hear the director of the Manhattan Project, Major General Leslie R. Groves, Jr., say words to the effect that the real purpose in making the bomb was to subdue the Soviets.

[22] In addition, FBI records show that in 1950, Rotblat's friend in Santa Fe was tracked down in California, and she flatly denied the story; the cheque had never been cashed and had been left to pay for items not available in the UK during the war.

[29][30] Rotblat returned to Britain to become senior lecturer and acting director of research in nuclear physics at the University of Liverpool.

Ewa, taking advantage of the fact that she was an ash blonde who, like Rotblat, spoke fluent Polish as well as Yiddish, smuggled the rest of the family out of the Warsaw Ghetto.

Displays of Polish anti-Semitism that she witnessed during the Warsaw Ghetto uprising embittered Ewa towards Poland, and she petitioned Rotblat to help the family emigrate to England.

Japanese scientists who had collected data from a fishing vessel, the Lucky Dragon, which had inadvertently been exposed to fallout, disagreed with this.

Rotblat was able to deduce that the bomb had three stages and showed that the fission phase at the end of the explosion increased the amount of radioactivity by forty times.

After the positive coverage of the manifesto, Cyrus Eaton offered to fund the influential Pugwash Conferences on Science and World Affairs, an international organisation that brought together scholars and public figures to work toward reducing the danger of armed conflict and to seek solutions to global security threats, particularly those related to nuclear warfare.

He successfully resisted a subsequent attempt to take over the conferences,[41] causing a Foreign Office official to write that "the difficulty is to get Prof. Rotblat to pay any attention to what we think ...

"[41] By the early 1960s the Ministry of Defence thought that the Pugwash Conferences were "now a very respectable organization", and the Foreign Office stated that it had "official blessing" and that any breakthrough may well originate at such gatherings.

[4] In parallel with the Pugwash Conferences, he joined with Albert Einstein, Robert Oppenheimer, Bertrand Russell and other concerned scientists to found the World Academy of Art and Science, which was proposed by them in the mid-1950s and formally constituted in 1960.

[43][44] As a result, for the first time in human history, a World Constituent Assembly convened to draft and adopt a Constitution for the Federation of Earth.

[40] During his tenure as president of the Pugwash conferences, Rotblat nominated Israeli nuclear technician Mordechai Vanunu for the Nobel Peace Prize every year from 1988 to 2004.

[56] His certificate of election to the Royal Society read He made important contributions to nuclear physics, both before and after working during the war on atomic energy problems at Liverpool and at Los Alamos.

This included observations on the angular distribution of protons from the (d,p) reaction, which led to an important tool for determining the spin and parity of nuclear levels.

A plaque commemorating Joseph Rotblat, unveiled in 2017 in the presence of Polish Ambassador Arkady Rzegocki, can be found outside the offices of British Pugwash, on the corner of Bury Place and Great Russell Street in London.