A "civilian" means someone who is not a member of the military. Some presidents have formerly served in the military, but as elected officials in the White House, they are serving as civilians in that office.
A prominent stand-off over the President's authority over the military occurred during the presidency of Dwight Eisenhower. Eisenhower had been a military general himself (during World War II), but then was elected to serve as civilian commander in chief as President of the United States. In 1950, when North Korean forces invaded South Korea,General Douglas MacArthur wanted to bomb the People's Republic of China and use Nationalist Chinese forces from Taiwan also in an attack on communist China. President Eisenhower would not grant these requests to escalate the war, and MacArthur made his opposition to Truman's policies a public argument. Eisenhower felt he had to relieve MacArthur of his command and maintain respect for civilian control of the military, with the President as Commander-in-Chief.
Okay, so obviously nuclear weapons proved to be a threat that would possibly destroy the world. The Soviet Union and the United States build up a stockpile of nuclear weapons and used the fact that the world may be destroyed to keep the other country at bay.
