Many essays have discussed this, and some documentaries have mentioned it!

Let me explain my personal point of view:

1. Einstein himself admitted that he hated war and opposed German Nazism during World War II. At that time, Germany's technology was very developed, and many scientists came from Germany. The military industry based on industry needs the support of high technology. Moreover, Germany has always had a soft spot for blitzkrieg, so Germany is not at all backward in armament quality. At that time, the U-submarine and the late "savior of German airspace"-He-178 jet at the end of World War II. The anti-fascist camp at that time urgently needed an overwhelming weapon to suppress fascist forces! Relativity provides a theoretical basis for the design of atomic bombs! Einstein should hold the hope of "achieving peace through violence", so he developed an atomic bomb for the United States!

Second, remember that he is a scientific genius. As a person obsessed with science (he later spent half his life studying the unified formula), it is natural to put knowledge into practice. At that time, it was impossible to try to develop nuclear weapons. Einstein certainly had a strong desire to realize it, so he was willing to develop an atomic bomb.

Third, from the big background, it seems that Germany had a heavy water production base in a mountainous area of Denmark during World War II, and an atomic bomb was about to be developed. It should be that British soldiers sneaked in in time and blew up its important facilities! From this perspective, the United States' eagerness to develop atomic bombs is also forced by fascist forces, and there is pressure to have motivation! Therefore, Einstein, who hates Nazism, will certainly not stand by!

To sum up here, it is purely a personal opinion, please adopt it carefully!