Come learn 2000+ kanji and 6000+ vocabulary words with WaniKani.
The middle help in the army or navy is the lieutenant colonel or the navy commander. I'm guessing these ranks are kind of in the middle or middle high end of the ranking scales?
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
The lieutenant colonel decided against immigrating to Japan.