"Moshe sent them . . . and Pinchas . . . and the sacred vessels." (31:6) The gemara explains that the "sacred vessels" included the Aron Hakodesh. Inside the Aron were housed the Shivrei Luchot - the remnants of the luchot that Moshe broke on the 17th Tammuz, when he witnessed the people sinning with the Golden Calf.
The great R' Yissachar Dov Rokeach z"l (the "Belzer Rebbe") asks: Why would the Bnei Yisrael take the shivrei luchot which were held in the Aron - a vivid reminder of the shameful sin of the Golden Calf - to the battlefront; a place in which they needed as many merits as possible to protect them!! Should we not fear that it will "testify" against Bnei Yisrael before the Heavenly Court & Chas V'Shalom cause defeat in war?!
He answers: Just the opposite - the broken luchot are in fact a constant reminder that no matter how low the Jewish people fall, Hashem always remains willing to accept their repentance and take them back, just as He did after the incident with the Golden Calf.http://www.shortvort.com/index.php?p=search&t=cat&s=Matos