"Good evening, Dr. Manhattan." (Chapter 1, Page 20, Panel 1)
The superhuman entity known as Dr. manhattan in Watchmen is a reference to the Manhattan Project of the mid-20th century. The Manhattan Project was the code name for the development of the atomic bomb right before the beginning of World War II. Since Dr. Manhattan was created within in intrinsic field lab that had to do with a lot of radiation, he is instantly associated with the atomic bomb. Moore's naming of his all-powerful being Dr. Manhattan shows that Manhattan is supposed to inspire not only security in the people of the Earth, but it needs to be shown that he is not truly human any longer and really more of a weapon than anything else. He is something detatched from humanity and whose human-given mission is only to destroy.