The relationships among nation states were fragile. At one point, the different states launched orbital nuclear rockets in response to other governments doing the same. Eventually, most governments had weapons in orbit. This almost destroyed the planet, but its ultimate destruction was avoided.
The aliens who employed Gary Seven were aware of the history of Omicron IV, as was Gary Seven himself. He compared the situation of Earth in 1968 to what happened on Omicron IV, which the Beta 5 computer agreed was similar. (TOS: "Assignment: Earth")