The Torah contains a series of commandments that relate directly to rules of warfare. Among them is the charge made by the High Priest – called mashu’ah milhamah – a priest anointed specifically for the purpose of leading the troops to war. The Torah commands that this priest instruct the soldiers – in Hebrew – that they cannot fear their enemies as they enter into war, and followed by listing those individuals who were free from participating as soldiers. Those exemptions included people who were in the middle of various uncompleted projects, e.g. someone who was building a house, planting a vineyard, or had gotten engaged to be married (see Devarim 20:1-9).
The Gemara concludes that the exemptions apply only to situations when the war is a milhemet reshut – a war of choice. Were it a milhemet mitzva – an obligatory war – such exemptions would not apply. According to the Meiri, a kohen mashu’ah milhamah was established in all times of war; during a milhemet mitzva only the first part of the speech, which were words of encouragement, was presented, and the exemptions were omitted.
According to the Mishna, the kohen mashu’ah milhamah warned the people of the noises and experiences of war that may disturb them, including the cries of the horses and clashing of swords, the banging of the shields and the pounding of the calgassim, the horns of war and shouts of people. The Sefer Be’er Sheva suggests that this is simply the interpretation of the tanna of the Mishna; the kohen mashu’ah milhamah simply read the verses that appear in the Torah. The Sefer HaHinukh, however, clearly understands that the kohen mashu’ah milhamah embellished the Torah’s description with these interpretations as well as his own.
Rashi and the Arukh translate calgassim as hordes of foot soldiers, whose marching sounds frightening. In fact, a calgas was the Latin word for shoe or boot, which, apparently, was used to indicate the soldiers themselves, as well.