For anyone new to the Johnson Valley/Hammer trails area: The Johnson Valley Shared Use Area, just north of the popular Hammer trails, is designated for military use up to two thirty-day periods per year. The 29 Palms military base uses it for training exercises so you'll need to be sure to avoid that area during those times.
Johnson Valley Shared Use Area Closure Schedule Updates
https://www.29palms.marines.mil/Staff-Offices/Government-and-External-Affairs/Johnson-Valley/The new boundaries of the Combat Center are identified with markers and warning signs for public safety. Individuals have a personal and legal responsibility to avoid trespassing onto the Combat Center regardless of the presence or absence of signs. This is consistent with all federal law and is not unique to the Combat Center. A full-size map of the current boundaries may be downloaded by clicking on the map below:
There's more information on the linked page, including an option to receive a text message when the access changes.