I am a former Army Officer who commissioned through the ROTC program. I graduated from college and served four years on active duty as an Infantry Platoon Leader.
The short answer to your question is no, it is not necessary to graduate from college if you plan on going into the military. If you do choose to forego college and go straight into the military you are limiting the roles in which you can serve. Basically, you can graduate from high school (or receive a GED) and enlist in one of the branches of service. A bachelor's degree or higher is required to become an officer (refer to the links below).
Prior to making this decision, you need to have a solid grasp of what you want to do in the military. Even if you do choose to go the enlisted route (I am by no means disparaging that path - there are tremendous career opportunities there and non-commissioned officers are absolutely crucial to each branch of service) the military values college education. I echo Zachary's sentiments here, if college is a feasible option for you, I would recommend pursuing a degree and then evaluating whether the military is right for you (including whether to go the officer vs. enlisted route).
I would recommend doing more research about what jobs officers and enlisted personnel perform in each branch and the steps required for different commissioning sources. Again, use the links below as a starting point.
I hope this helps and good luck - feel free to reach out if you have any additional questions.
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I don't think it is a question of "necessary" or "not necessary" -- it is completely your choice. There may be certain jobs in the military that you may only qualify for with a college degree. I am a big proponent of going to college, as I feel it provides a great foundation of education and independence for your future; however, it is important to look at the cost vs. benefit of the degree. Consider what the cost of this investment (college education) you are making in yourself and what the earning potential is in the field you want to enter. The "student debt crisis" in this country is perpetually growing. Additionally, consider what you do after the military (if you choose to retire from military service after x years), may have a 4 year degree as a qualification. Feel free to message me if you have any other questions.
I agree with Zachary! However, you do have the option of attending a college or university and joining ROTC. Upon graduation you would graduate as a commissioned officer ( Second Lieutenant I believe). Although, going straight to the military would allow you to pick up training for a future job in the states without having to make a career "of it" as you would if you were a an officer. You would not spend as much time in the military!