In the United States, while some branches of the military may have specific requirements for education level or completion of a high school diploma or General Educational Development (GED) certificate, the general requirement is that individuals must have a high school diploma or equivalent to enlist.
Therefore, it is generally necessary to have at least a high school diploma or GED to join the military. However, it's always best to check with the specific branch of the military you are interested in for their current requirements and any exceptions or alternatives they may offer.