My source may not be entirely accurate as it comes from accounts made by various ROTC cadets but getting into ROTC during your junior year now is extremely hard. AFROTC had discontinued the additional week in Summer Field Training for any new juniors that would have to waive the first two years. The Army LDAC is very competitive and with fewer slots AROTC units are more inclined to give the slots to cadets that have been in the program for some time rather than a newcomer. The only options I can see for an upcoming junior is to apply for OCS or OTS which is very competitive in its own nature or take an additional year to allow time for you to enroll as if you were just a sophomore (Not sure if it would work but it sounds logical).
If you are an upcoming sophomore, you will have to take the accelerated path which means to take both the first year and second year course. I'm not sure what you mean without going through a program but you do have to go through some kind of summer officer training to earn a commission into the military. If you are an upcoming sophomore rather than a junior, take initiative and be competitive when it comes to getting a slot in summer field training or LDAC.