The modern Hebrew word for soldier is chayal, coined by Eliezer Ben Yehuda. However, the biblical term for solider is ish tzavah, a man of the army (eg Numbers 31:21).
Just as the biblical ish adamah, man of the earth (Genesis 9:20), gave way to one word, Aramaic terms ikkar and haklai, to designate a farmer, Ben Yehuda sought a one word term for soldier. He looked to the root ch-y-l.
Ch-y-l has several related meanings: strength, wealth and an army of fighters. Thus the young David is described as doing "chayil defeating the Amalekites" (I Samuel 14:48). Chayil here denotes strength, success, and military might.
Later Jesse asks his youngest son to take food to his brothers who were battling the Philistines, led by Goliath (I Samuel 17). David arrives just as the army, the chayil, are setting out.
Today tzavah refers to the army, but the root ch-y-l is used for different branches for the military. Chel Ha'avir is the Air Force, and Chel Hayam is the navy.
Last winter television screens around the world showed the advanced field hospital set up in Haiti by the IDF's Chel Harefuah - medical corps.