In Israel's, at the Exodus, every man above 20 was a soldier (Numbers 1:3); each tribe a battalion, with its own banner and leader (Numbers 2:2; Numbers 10:5-6; Numbers 10:14). Their positions in camp and on march were accurately fixed. The whole host moved according to preappointed alarms on the trumpet. So (Exodus 13:18) they "went up harnessed" (margin five in a rank; chamushim, from chameesh, "five"; or from chomesh, "the loins," with the loins girt), prepared for the march, not fleeing away as fugitives. Five was a number regarded as inauspicious by the Egyptians, but honored by Israel; witness the five books of the pentateuch, the Jubilee of fifty years. Manetho describes the Israelites as 250,000 lepers, five X fifty thousand. The exactness of their martial order is implied in Balaam's metaphors (Numbers 24:6).
The "scribe of the host" made the conscription and chose the officers when needful (Deuteronomy 20:5-9; 2 Kings 25:19; 2 Chronicles 26:11). The army was divided into thousands and hundreds with captains over each; the family too was respected in the army organization, as being the unit in the Jewish polity (Numbers 2:34; Numbers 31:14). Before the time of the kings their tactics were of a loose desultory kind; but the kings established a body guard, the first step toward a standing army. Saul had 3000 picked men (1 Samuel 13:2; 1 Samuel 14:52; 1 Samuel 24:2). David had 600 before his accession (1 Samuel 23:13); after it he added the Cherethites and Pelethites and Gittites (2 Samuel 8:18; 2 Samuel 15:18), and veteran guards (shalishim, "captains," 1 Chronicles 12:18; Ezekiel 23:15; Ezekiel 23:23, "princes," "great lords") whose "chief" was about David's person as adjutant. He called out also monthly a regiment of national militia, twelve regiments in all, under officers (1 Chronicles 27:1).
A "captain of the host," or commander in chief, led the army in time of war; as Abner under Saul, Joab under David. Judaea and the northern kingdom Israel being hilly, were little suited for chariots and horsemen, except in the plains of Esdraelon and Philistia, and toward Egypt and Syria. Moreover, God had forbidden the multiplication of horses (Deuteronomy 17:16). But their own unfaithfulness exposed them to the enemy's powerful chariots; so they too longed to have similar ones (Joshua 17:16; Joshua 11:9; Judges 1:19; Judges 4:2; 1 Samuel 13:5). David reserved 100 from the Syrian spoils (2 Samuel 8:4). Solomon afterward largely increased the number from Egypt (1 Kings 10:26-29; 1 Kings 9:19); in all 1400 chariots, 12000 horsemen. The grades in the army appear in 1 Kings 9:22, "men of war" (privates), servants (subalterns), princes (captains), captains (staff officers), rulers of chariots and horsemen (cavalry officers).
The body guard was permanently maintained (1 Kings 14:28), the militia only exceptionally called out. The Syrians reduced the cavalry to a mere fragment in Jehoahaz's reign. Jotham in Judah had a large cavalry force (Isaiah 2:7), but it was much brought down in Hezekiah's reign, so that the Jews, in violation of God's prohibition (Deuteronomy 17:16), looked to Egypt for horses and chariots (Isaiah 31:1; Isaiah 36:9; Psalm 20:7). In action the army was often in three divisions (Judges 7:16; 1 Samuel 11:11; 2 Samuel 18:2). Jehoshaphat divided his into five bodies (answering to the five geographical divisions then), but virtually Judah's heavy armed men formed the main army, the two light armed divisions of Benjamin the subsidiary bodies. At the Exodus the number of soldiers was 600,000 (Exodus 12:37), at the borders of Canaan 601,730; under David, 1,300,000 men capable of service, namely, 800,000 for Israel, 500,000 for Judah (2 Samuel 24:9), but in 1 Chronicles 21:5-6 it is 1,570,000; namely, 1,100,000 for Israel, and 470,000 for Judah.
The discrepancy is due to the census having been broken off (1 Chronicles 27:24). The militia (1 Chronicles 27:1, etc.), 288,000, was probably included in Chronicles, not in Samuel. The exact census was not entered in the annals of the kingdom (1 Chronicles 27:24); hence the amount is given in round and not exact numbers. Levi and Benjamin were not reckoned, the latter owing to Joab's repugnance to the census (1 Chronicles 21:6). Jehoshaphat's army was 1,160,000 (2 Chronicles 17:14-18). John Hyrcanus first introduced mercenaries. The Roman army was divided into legions, each under six tribunes ("chief captains," chiliarchs, Acts 21:31), who commanded in turn. The legion had 10 cohorts ("bands," speira, Acts 10:1), the cohort into three maniples, the maniple into two centuries (each 100 men originally), commanded by a centurion (Acts 10:1-22; Matthew 8:5).
The "Italian band" or cohort consisted of volunteers from Italy, perhaps the procurator's body guard. "Augustus' band" or cohort (Acts 27:1) were either volunteers from Sebaste, or a cohort similar to "the Augustan legion." Caesarea was the Roman head quarters in Israel. The ordinary guard was a quaternion of four soldiers, answering to the four watches of the night, and relieving each other every three hours (Acts 12:4; John 19:23). Two watched outside a prisoner's door, two inside (Acts 12:6). "The captain of the guard" (Acts 28:16) was probably commander of the Praetorian guards, to whom prisoners from the provinces were committed. The "spearmen" (dexiolabi; Acts 23:23) were light armed body guards, literally "protecting the right side," or else "grasping the weapon with the right hand."
Fausset, Andrew Robert M.A., D.D., "Definition for 'army' Fausset's Bible Dictionary".