Parshas Chukas:
?? ?????–??? ???? ??? ?????, ???; ???? ????, ???? ???? ??????, ???? ??-??-???? ????, ??-????. ??-?? ????? ??????, ??? ?????;
The Gemarra in Bava Basra Explains this Passuk:
?”? ????? ?? ???? ?”? ????? ??? ????? (????? ??) ?? ?? ????? ??????? ???’ ??????? ??? ??????? ????? ???? ????? ???? ????? ?????? ?? ???? ???? ???? ???? ???? ???? ????? ???? ?????
How can we understand how chazal make a such a drush from this passuk? (That this refers to the yetzer hara?)
R’ Yonasan Eibushitz explains to us what the city of “Cheshbon” means. It was a simple city in Moav. Had the king of Moav properly protected it with guards, it never would have fallen into the hands of Sichon. However, since it was neither a large nor an important city, therefore the king of Moav did not feel the need to protect it as well as he could have. Thus it fell, quite easily, into the hands of Sichon (which enabled him to conquer all of Moav).
Chazal tell us (in the above quoted Bava Basra) that this is identical to our battles with the Yetzer Hara. If the yetzer hara tries to get us, even on a “mitzva kalla” or a seemingly little aveira, we still must fight with all our strength, lest our yetzer win. For if it wins, it becomes much easier for it to continue winning (just like Sichon vs. Moav).