The letter J on the map represents?

the answer is in the attached picture

1. Spies gather information on Manhattan Project
2. American creates 3 bombs in Manhattan Project
3. USSR detonates first atomic bomb
4. US detonates hydrogen bomb.

The USSR began gathering intelligence on Britain and the US starting in 1941. Soviet spies were able to get information on the Manhattan Project before the US FBI even knew about it. The US will detonate 3 bombs--1 a test and 2 in Japan in 1945. The USSR will then detonate their atomic bomb in 1949--sooner than any American would have suspected (thank you Soviet spies). Lastly, in 1952, the US detonated the hydrogen bomb.