Why We Recite Shema Before a Bris

The Imrei Chaim of Vizhnitz once took part in a vacht nacht (a seudah held on the night before a bris milah to protect the baby boy, with divrei Torah and keriyas Shema). After all those present had recited keriyas Shema as is customary, the Rebbe told over the following. Rav Pinchos of Koretz once approached Rav Mordechai, the chazzan of Zaslov, and asked him, "You perform at many simchas and recite Shema at many vacht nachts. Tell me, why do we recite keriyas Shema on the night before the bris?" The chazzan answered with the question of Chazal (Berachos 13a): why does the parsha of Shema come before the parsha of V'haya im shmoa? Because first we accept upon ourselves the yoke of the Kingship of Heaven, and only afterwards the yoke of the mitzvos. The same reasoning explains the custom. Since the baby will enter the yoke of the covenant of mitzvos tomorrow, we first accept upon ourselves the yoke of Hashem's Kingship, and so we recite Shema on the night before the bris.