Skynet computes John Connor as being the largest threat to their existence, there-fore Skynet's primary mission is to terminate John Connor. Connor avoids all attempts of assassinations in real time forcing the Machines to resort to "time-travel" to kill him. They first send a T-800 to kill his mother Sarah Connor to prevent his birth, in failing that they later send a T-1000 to kill him in his teen years, however Tech-com sent a reprogrammed T-800 to protect Connor. They failed again in terminating Connor and he continued his life on the run.
A third terminator, this time a T-X is sent back in time to kill Connor and all else who will be relevant to the upcoming war, including his future wife Kate Brewster. Again a reprogrammed T-850 is sent back to protect him. The T-X uploads the virus that corrupts Skynet's mainframe causing Judgment Day however fails in terminating John Connor. The earth is then wiped out from an attack on itself (from Skynet), John Connor survives the attack and leads the rebellion against the machines.
John has proven himself, time and time again to be a natural born leader. Though he once tried to run from his future as leader of the resistance somehow he was always set back on course. No matter how hard he tried to avoid the future and the past they were always knocking on his door. Now John has the chance to prevent the future and he is doing everything in his power to destroy Skynet and save the world from itself.
The war between man and machines has already started. John Connor learns that the machines are capturing people so that they can replicate human tissue. He must rush to save the young Kyle Reese and destroy any machines that get in his way.
Terminator: The Sarah Connor Chronicles
In the live action television show Terminator: The Sarah Connor Chronicles a teenage John is portrayed by Thomas Dekker. He and his mother, along with the help of Cameron (a good terminator sent back by John himself) have time traveled to the year 2007 in hopes of destroying Skynet before it is even created. Though he's not the military trained expert of the future, John handles himself quite well in dangerous situations.
Unlike his mother, John trusts Cameron more fully as he has not yet had any reason to doubt her commitment to their cause.