From the Swiss Guards' Page at the Vatican website:
On the morning of May 6th, 1527, from his headquarters set up in St. Onofrio's Convent on the Gianicolo hill, Captain General Bourbon launched a series of attacks on Rome. During one of them, at the Torrione Gate, while leading the assault of the walls, he himself was mortally wounded. After just a moment's hesitation, the Spanish mercenaries broke through the Torrione Gate, while the lansquenets invaded the road of Borgo Santo Spirito and St. Peter's. The Swiss Guard, standing firm at the foot of the obelisk (now in St. Peter's Square, but then near the German cemetery within the Vatican close to the Basilica), together with the few remnants of the Roman troops, resisted desperately. Their Captain, Kaspar Röist was wounded, and later killed by the Spaniards in his quarters in front of his wife, Elizabeth Klingler. Of the 189 Swiss Guards, only 42 survived, the ones who, when all was lost, under the command of Hercules Göldli guarded Clement VII’s retreat to safety in Castel Sant’Angelo. The rest fell gloriously, massacred together with two hundred fugitives, on the steps of the High Altar in St. Peter's Basilica. Pope Clement VII and his men were able to escape to safety, thanks to the "Passetto", a secret corridor which Pope Alexander VI had built along the top of the wall connecting the Vatican with Castel Sant’Angelo. The savage horde was in a hurry, for fear that the League troups would cut off their retreat. Across the Sisto bridge the lansquenets and Spaniards fell on the city and for eight days committed every sort of violence, theft, sacrilege and massacre, even the tombs of the Popes, including that of Julius II, were violated in search of spoils. There were as many as 12 thousand dead and the booty amounted to ten million ducats. All that happened cannot really be regarded with surprise because the imperial army and in particular Frundsberg's lansquenets, were animated by a violent spirit of crusade against the Pope. In front of Castel Sant’Angelo where the Pope had retreated, a parody of a religious procession was set up, in which Clement was asked to cede the sails and oars of the "Navicella" (boat of Peter) to Luther, and the angry soldiery shouted "Vivat Lutherus pontifex!" (Long live Luther, Pontiff!) The name of Luther was incised with the tip of a sword across the painting of the "Dispute of the Most Holy Sacrament" in the Rooms of Raffaello, out of disdain, while on another wall a graffito hailed Charles V, emperor. Concise and exact was the description given by the Prior of the Canons of St. Augustine at that time: "Mali fuere Germani, pejores Itali, Hispani vero pessimi." (The Germans were bad, the Italians were worse, the Spaniards were the worst.) Besides the irreplaceable damage of the destruction of the relics, during the Sack of Rome, inestimable art treasures, namely the greater part of the Church's finest artisan-made gold and silver ware, were lost forever. On June 5th, Clement had to surrender and to accept heavy conditions: he had to cede the fortresses of Ostia, Civitavecchia, and Civita Castellana, to hand over the cities of Modena, Parma and Piacenza, and to pay the sum of four thousand ducats. Moreover, a ransom for the freedom of prisoners was demanded. The papal garrison was replaced by four companies of Germans and Spaniards, and two hundred lansquenets took the place of the Swiss Guard which had been suppressed. The Pope obtained permission for the surviving Swiss Guards to join the new Guard, but only 12 of them accepted, among them Hans Gutenberg of Chur and Albert Rosin of Zurich. The others wished to have nothing to do with the hated lansquenets.This is the oath the new gaurdsmen, called Halbardiers, take on this day:
I swear I will faithfully, loyally and honourably serve the Supreme Pontiff John Paul II and his legitimate successors, and also dedicate myself to them with all my strength, sacrificing if necessary also my life to defend them. I assume this same commitment with regard to the Sacred College of Cardinals whenever the See is vacant.
Furthermore I promise to the Commanding Captain and my other superiors, respect, fidelity and obedience. This I swear! May God and our Holy Patrons assist me!
The LD salutes the new guardsmen, and wishes them all the best in their service.