1st King ' s Immemorial Infantry Regiment of AHQ The King ' s Own Immemorial 1st Infantry Regiment of the Spanish Army is considered to be the oldest armed unit in the world . There is certain proof of this and according to what the chronicles mention , King Ferdinand III , " The Saint , " in 1248 , during the conquest of Seville , with some of his men - at - arms , assaulted and took a tower . Seemingly , with such boldness and bravery they gained the admiration of the King . The campaign finished and with the consequent disbandment of troops , King Ferdinand decided to permanently keep with him said force , giving origin to the permanence of the Armies , that is to say , the origin of itself . On 28 August 1632 , on the initiative of Philip IV , a special Corps of troops was ordered to be formed with veteran soldiers , reenlistments and Knights of noble ancestry - men so skilled that they only took arms when the Monarch assumed command in person , designating it " King ' s Guard Colonelship . " Its organization commenced in Almansa in 1634 , its force fixed at fifteen companies with 90 harquebusier s , 40 musketeer s , and 60 corslet s and pikemen each . Later , in 1638 , it was augmented to twenty . Its first Colonel , Count Duke of Olivares , Don Gaspar de Guzmán , whose post was the recently created " King ' s Guard Colonelship , " was in command of very distinguished soldiers of great renown . In 1640 , Philip IV elevated it to the category of King ' s Guard Regiment and in 1664 to Castile Tercio . From 1701 to 1710 it was dedicated to its ancient mission , guarding the Monarch , and it was in 1707 when it received the designation Castile , and in 1710 that of Castile Immemorial Infantry Regiment . It gained glory in the Spanish War of Succession and the Italian Campaigns ( 1718 - 1749 ) , eventually it received the name King ' s Infantry Regiment on 7 January 1766 . That same year , King Charles III , persuaded by the antiquity of the Regiment , declared it the King ' s Immemorial . After diverse historical changes in losing and receiving its name , it received its present designation in 1995 . Traditionally , the Regiment is linked to the Spanish Royal Household , all the Princes of Asturias since 1862 are enrolled as Honorary soldiers in the 1st Company . Following the tradition Don Felipe de Borbón y Grecia ( Prince of Asturias and heir to the Crown ) assumed his post as a soldier in 1977 . Traditional uniform . Instead of uniformity and as what is practiced in other countries with a military tradition , in Spain as well , King Juan Carlos I in 1981 accepted the proposal of CSA to equip two companies , which could render honors and participate in established ceremonies special in character , with the uniform and armaments of the end of the reign of Charles III . The period chosen was that from 1780 to 1786 , during which in accordance with the organization of Spanish Infantry , each Regiment had two Battalions , each one formed of nine companies , one of grenadiers and eight of fusiliers . The description in detail of the uniform is as follows : Fusilier uniform . It had a white uniform , with the green emblem , that of the King , by special grace it was dressed elegantly in the colors of the Royal Household troops , and as a special grace they wore " botoneras " on the vest and dress coat . Grenadier uniform . To wear this uniform there existed a norm in the selection of grenadier personnel , they must not wear glasses nor a beard ; now it is possible to wear a beard ( they are the only ones , together with the dragoons , who have the privilege to wear beards ) . In this era the grenadier company was considered to be a distinction or preference usually chosen over the fusilier companies by the bravest and tallest soldiers . These troops chosen not only for their height , but also for their valor and bravery , posted at the head of the Regiment , escort the Colors and are the first in the trenches . As a vanguard and reconnaissance force , they wear a short saber . The grenadier cap augmented their stature and the brass " mechero " on the strap of the cartridge pouch , in the same way the " sardinetas " which they wore on their cuffs distinguished these preferred companies of the Infantry . Current functions and structure . At present the Regiment is the Unit responsible for providing the central seat of Army Headquarters security , services , and support needed for its functioning , it also helps give security to that which the authorities decide . These functions support the fundamental objective of facilitating the functioning of the different Centers and Organisms of which the Headquarters is composed , to be able to achieve the following tasks : The Regiment in order to execute these functions is organized , in addition to the Command Staff , in the following Units : In addition to its usual functions it has participated , when it was required , in actions in behalf of the civilian population ( ex . Environmental cleaning carried out in Galicia because of the sinking of the Prestige , etc . )