http://revach.net/article.php?id=2515

Section: Parshas HaShavua   Category: Chasidus
Parshas Matos: Imrei Emes - The Shevet Without Any Emotional Baggage

Did Shevet Levi Send Soldiers to Fight With Midyan or Not? The pasuk tells us that each Shevet sent 1,000 soldiers to fight the war with Midyan.  Rashi says that the words (31:4) "L'Chol Matos Yisroel" teach us that even Shevet Levi sent men.  However in the next pasuk it says "VaYimasru MeiAlfei Yisroel Elef LaMateh Shnaim Esrei Elef", there were a grand total of 12,000.  With Levi there should have been 13,000?

The Gerrer Rebbe Rav Avrohom Mordechai Alter (Imrei Emes) answers that Rashi says on this pasuk says that the Torah uses the word "VaYimasru" because the soldiers had to be forced to go to battle, since they were reluctant, knowing that after the war Moshe would die.  However Shevet Levi was the Shevet that had no compunctions when it came to doing Hashem's decree even at the expense of killing there own family, as we see by the Eigel and in Parshas Devarim by Moshe's Bracha to Levi (HaOmer L'Aviv U'L'Imo Lo R'eesiv).  Therefore they went willingly without having to be forced.  So while the total of soldiers was really 13,000, the pasuk says that 12,000 had to be forced to go.  (Iturei Torah)