Complex sentence: The lower-enlisted (in the United States Army, E- 4 and below) are the core of the working force, while the NCO corps (in the United States Army, E- 5 and above) provides the direct leadership and training to these soldiers.
Split into simpler sentences: The lower-enlisted in the United States Army (E- 4 and below) are the core of the working force. While the NCO Corps, made up of E- 5 and above, provides the direct leadership and training to these soldiers.