The United States made their first effort in the direction of Fission Process with the encouragement of Albert Einstein. G.B. Pegram of (Columbia University) arranged this historical conference between Fermi and the Naval department in March 1939,where the entire team including Einstein made the first official contact with the White House.

Here are the alleged events as they unfolded:

1. 1941 - Urey and Pegram visit England to set cooperative efforts in the experiment.

On  6 December 1941, the project was placed under the direction of the Office of the Scientific Research & Development (OSRD) and the managing officials were J.B. Conant, Briggs, Pegram, A.H. Compton, E.O. Lawrence, H.C. Urey, E.V. Murphree, H.T. Wensel, S.K. Allison, J.W. Beams, G. Breit. E.U. Condon and H.D.Smyth.

To accelerate their activities on the bomb-making experiment, the U.S. Department organized the Manhattan district to carry forward the work. In September 1942, Brig. Gen. L.R. Groves was appointed as head of the project for all army related activities

2. In early 1942, an experiment was carried out in the United States about the possibility of super explosive and the data used were less than 100 lb of U²³⁵ or Pu²³⁹ releasing an energy equivalent to about 20,000 tons of TNT.  At that time, less than 0.000001 lb of this material was produced, while there was hardly any knowledge of quantity production process.

In 1942 itself, the then U.S. Government allotted necessary funds on priority basis to start the research work under the supervision of a committee headed by L.J.Briggs, Director of National Bureau of Standards. To implement this proposal a proper budget was allotted. 

3. 1943 - A combined policy committee with Great Britain was formed and several noted scientists proceeded to United States to join  the combined project.

4. 1945 - In short, with the passage of time, atom bombs were manufactured and the first atomic bomb was exploded at 5.30 A.M. on July 16, 1945, at a site of the Alamogordo air base 120 mi. S. of Albuquerque, N.M. 

As per record available on the subject, it was detonated on top of a steel tower surrounded by scientific equipment, with remote monitoring taking place in bunkers occupied by scientists and a few dignitaries positioned there, were kept about 10,000 yards away.

On August 6 and 9, 1945, the cities of Hiroshima and Nagasaki were destroyed by the first atomic bombs used in warfare.

5. 1951 - In early 1950s, the U.S. invented another deadlier fusion bomb known as Hydrogen Bomb or Thermonuclear Bomb.  In May 1951, U.S. tested this bomb and on 1 November 1952 conducted another test on megatons of TNT (1 megaton is equal to 1,000,000 tons).

The explosion resulted in the disappearance of the entire island and created a hole in its place, 175 ft. deep and a mile in diameter.

6. 1954 - The next test was conducted on 1 March 1954, which was equivalent to 12 to 14 megatons explosive power. This test was so serious that it severely exposed 23 Japanese fishermen to radioactive ash who were 70 to 90 miles away from the explosion.

7. 1957 - The race started by other countries, the US conducted the test on 12 August 1953 followed by a British test at Christmas Island on 15 May 1957.

 However, a treaty was signed on 5 March 1970 banning the spread of nuclear weapons to non-nuclear powers.

Other powers as on today apart from already mentioned above are China, France, India, Pakistan and North Korea. Israel is also believed not only to have possessed all these capabilities but also holds requisite stockpile, which it has reserved to use only in emergency.

 There are other countries including Iran, which are trying to join the race of obtaining the destruction of nuclear capability. UK, France and Germany have persuaded Iran to stop nuclear experiments. US has also warned Iran to  refrain from  nuclear experiments, to avoid military action against it. Negotiations with North Korea are also going on to reduce the spread of nuclear arms.