Due to the secrecy surrounding atomic fission research only vague warnings could be given in leaflets dropped by Allied bombers. Even if a warning was explicitly given since the technology to create such a weapon was largely unknown to the population and no such kind of explosive was known to closely resemble an atomic bomb most would have regarded it as war propaganda. Einstein's E=MC^2 is what gives nuclear reactions more energy than chemical reactions but most Japanese like the Germans in the early 1940s would have dismissed Einstein's work as "Jewish science".

According to the Atomic Heritage Foundation website the main text of the Hiroshima warning leaflets was:

"TO THE JAPANESE PEOPLE:

America asks that you take immediate heed of what we say on this leaflet.

We are in possession of the most destructive explosive ever devised by man. A single one of our newly developed atomic bombs is actually the equivalent in explosive power to what 2000 of our giant B-29s can carry on a single mission. This awful fact is one for you to ponder and we solemnly assure you it is grimly accurate.

We have just begun to use this weapon against your homeland. If you still have any doubt, make inquiry as to what happened to Hiroshima when just one atomic bomb fell on that city.

Before using this bomb to destroy every resource of the military by which they are prolonging this useless war, we ask that you now petition the Emperor to end the war. Our president has outlined for you the thirteen consequences of an honorable surrender. We urge that you accept these consequences and begin the work of building a new, better and peace-loving Japan.

You should take steps now to cease military resistance. Otherwise, we shall resolutely employ this bomb and all our other superior weapons to promptly and forcefully end the war.

EVACUATE YOUR CITIES.

ATTENTION JAPANESE PEOPLE. EVACUATE YOUR CITIES.

Because your military leaders have rejected the thirteen part surrender declaration, two momentous events have occurred in the last few days.

The Soviet Union, because of this rejection on the part of the military has notified your Ambassador Sato that it has declared war on your nation. Thus, all powerful countries of the world are now at war with you.

Also, because of your leaders' refusal to accept the surrender declaration that would enable Japan to honorably end this useless war, we have employed our atomic bomb.

A single one of our newly developed atomic bombs is actually the equivalent in explosive power to what 2000 of our giant B-29s could have carried on a single mission. Radio Tokyo has told you that with the first use of this weapon of total destruction, Hiroshima was virtually destroyed.

Before we use this bomb again and again to destroy every resource of the military by which they are prolonging this useless war, petition the emperor now to end the war. Our president has outlined for you the thirteen consequences of an honorable surrender. We urge that you accept these consequences and begin the work of building a new, better, and peace-loving Japan.

Act at once or we shall resolutely employ this bomb and all our other superior weapons to promptly and forcefully end the war.

EVACUATE YOUR CITIES."

Source:  https://www.atomicheritage.org/key-documents/warning-leaflets

Not believing an atom bomb was possible before 1945 would have been reasonable. The science fiction author H.G. Wells had fictionalized atomic weapons decades earlier and the scientist Ernest Rutherford described the idea of arnessing atomic power as "moonshine".

• history
• science
• education