http://revach.net/article.php?id=3779

Section: Parshas HaShavua   Category: Quick Vort
Parshas Bamidbar: Rav Shamshon Refoel Hirsch - Who Is Fit For The Jewish Army?
When counting Klal Yisrael, it is always the men between the ages of twenty to sixty who are counted, because they are fit to serve in the army.  Yet, we see that when counting, there is great emphasis placed on counting the people in family groups.  Each person must account for his yichus, lineage.  

The Iturei Torah brings from Rav Shamshon Rephael Hirsch that while in most armies, the best soldiers are battle hardened, brave, and strong warriors accustomed to the savagery of war, this is not so in the army of Hashem.  Hashem's army is best served by compassionate, merciful souls who do not enjoy murder.  Even though in war a good soldier must kill when commanded to do so, however for a Jewish soldier, killing is not be an instinctive act.  

In Kedushin (76) it says that anyone who fought in Dovid HaMelech's army, did not have his yichus  questioned.   The most important background check before admitting someone into the army is to check the kind of family he comes from.  People from good, stable, respectable families are the kind of soldiers Am Yisroel needs to fight Milchemes Hashem and be Mikadesh Shem Shamayim.