John Connor leads the resistance from Los Angeles in Terminator Salvation.
The film presents Los Angeles as a devastated battleground after Judgment Day. Human survivors fight Skynet’s machines across ruined urban and industrial areas, while the resistance coordinates operations against the machine network. John’s position is important because he is recognized as a major future leader, although the film also follows Marcus Wright’s separate journey.
The location connects the future-war story to the earlier films, which are also strongly associated with Los Angeles. The first three theatrical films use the city as a major setting for Sarah and John’s attempts to prevent or survive the rise of Skynet.
This question refers to the resistance headquarters and operational setting in the film, not to every location visited during the story. Terminator Salvation also includes sequences in other areas of the post-apocalyptic American landscape, but Los Angeles is the named city most closely tied to John’s command.