You might like to read one of the scrolls found near Qumran on the Dead Sea. It is called "The War Scroll." It describes the victorious battle of the sons of light over the sons of darkness, the Kittim, in open battle. In it, the role of the priests and levites is given as such: 9"When the battle lines are arrayed against the enemy -battle line against battle line - there shall go from the middle opening into the gap between the battle line seven (10), priests of the sons of Aaron, dressed in fine white linen garments: a linen tunic and linen breeches, and girded with a linen sash of twined fine linen, violet, (11)purple and crimsom, and a varicolored design, the work of skilled morkman, and decorated hats on their heads...The one priest shall walk before all the men of the battle line to encourage them for battle... seven levites will go out with them...In their hands shall be the seven trumpets of rams' horns."
So you could see that the priests and the levites were to lead the army into the open field where they would face the enemy. The War Scroll discusses tactics and envisages an Israelite Army equally large and as well equiped as the enemy. And, I might add, winning in the end.
Now from reading Josephus, we see that such a large scale battle never occurred. The battles of Jotopata and Jerusalem were sieges, and the smaller battles seem more like Roman massacres. In any case, the Israelites were either totally unprepared for the war, or simply did not have a professional army like the one described in the War Scroll. The answer is probably both.
Hope this helps!
© Charles N. Pope, US Library of Congress. All rights reserved.