Infantry was the largest branch of the Persian army. It consisted of three parts: the King's bodyguards, the Immortals, and troops from allied peoples and satrapies of the Persian Empire. During a military campaign, 2,000 royal guards always escorted the King. The bodyguards were the most distinguished and valorous Persian warriors. Another unit of the Persian elite infantry was the Immortals. There were invariably 10,000 of them. If any of the Immortals was killed or incapacitated, he was immediately replaced by warriors chosen from the other troops. Therefore, their number always remained unchanged. Many Persian infantrymen wore huge but light shields woven from twigs, which gave good protection from arrows. A considerable part of the Persian infantry used bows. Hiding behind large shields, archers shot at the enemy, inflicting considerable losses.