After smaller battles in the Außerfern and at the Porta Claudia in Scharnitz near Seefeld, the Cactus Division of the US armed forces stood in Zirl at the gates of the Gau capital Innsbruck on 3 May 1945. A handful of resistance fighters led by Fritz Molden and the later Tyrolean governor Karl Gruber had occupied barracks and official facilities in Innsbruck after the Gau leadership, Gestapo and SS had fled the scene. Nevertheless, the GI's did not know what to expect in Innsbruck, as Adolf Hitler had declared Tyrol to be part of the Alpine fortress, the retreat that was to be defended to the last man. If Innsbruck were to become a battlefield, as had been the case in many cities, this would result in the destruction of the city. The fact that it did not come to that and Innsbruck was surrendered without a fight is due to a group of young people who were involved in the US espionage operation Operation Greenup laid the foundations for peaceful capitulation.

The male protagonists of this cinematic coup were Friedrich "Fred" Mayer, Hans Wijnberg, Franz Weber and Anna Niederkircher. The two Jews Mayer and Wijnberg had landed in New York while fleeing National Socialism. They had volunteered for service in Europe and were deployed with the OSS, the US military intelligence service. Weber had been stranded in a prison camp in southern Italy as a deserter from the Wehrmacht. After his war experiences, the staunch Catholic wanted to help overthrow the Nazi regime in his Tyrolean homeland. Together, they were to spy on the supply line over the Brenner Pass from Innsbruck as well as war-relevant infrastructure and industry such as the Messeschmitt factories in Kematen.

On 26 February, the three men and their equipment were dropped by plane over the Ötztal Alps in the wintery high mountains. Using sledges and public transport, they made their way to Oberperfuß, Franz Weber's home village, in the middle of enemy territory with all their equipment. Here they did not encounter Hitler's feared Alpine fortress, but rather support from the community of Oberperfuß, which had always been strictly Catholic, conservative and critical of the regime. Above all, those close to Weber, his sisters Eva, Margarete and Luise, his neighbour Maria Hörtnagl, but above all his fiancée Anni Niederkircher and her mother Anna, the landlady of the Gasthof zur Krone, played invaluable roles in providing supplies, camouflage and accommodation.

Franz Weber was the group's local guide. Fred Mayer mingled with the population in Oberperfuß, Innsbruck and Kematen under various identities, as a Wehrmacht soldier in the officers' mess, as a worker at the Messerschmitt factories or as a French forced labourer. He forged links with other resistance groups and gathered information. Weber's sisters harboured him and provided him with all sorts of things, such as forged papers or a stolen Wehrmacht uniform. Anni Niederkircher was the link between Oberperfuß and Innsbruck. Hans Wijnberg, as a radio operator, maintained communication with the US army base in Bari.

Everyone knew that if their risky operation was discovered, they and their families would be condemned to death. This happened at the end of April. Robert Moser, the radio dealer and resister who had employed Fred Mayer in his shop, was exposed. He was interrogated, tortured and finally beaten and whipped to death at the Gestapo headquarters in Innsbruck's Herrengasse. On 20 April, Fred Mayer was also arrested and tortured in Herrengasse. But he held out, and even more: after revealing himself to be a member of the US secret service, he was able to negotiate with Gauleiter Hofer to have Innsbruck handed over as a free city without a fight. In return for Mayer's assurance that he would be treated as a prisoner of war, Hofer issued an order to the population in a radio address on 2 May to refrain from any fighting.

At 2 pm on 3 May, Fred Mayer, still scarred from his treatment by the Gestapo, reached the US troops near Zirl with this message. A few hours later, the ceasefire came into effect. The vehicles and soldiers were able to enter the town without further bloodshed and destruction.

The memory of the Operation Greenup and the heroic actions of all those involved under extreme danger were not remembered for a long time in favour of the story of self-liberation by the brave Tyrolean people. It was not until 2010 that Fred Mayer, who was honoured with the Purple Heart who had received the US military's highest medal for valour, was honoured by the state of Tyrol late but still at the age of almost 90. Hans Wijnberg received a Medal of Merit from the City of Innsbruck ten years after his death. Franz Weber, who served as a member of the provincial and national councils after the war, was honoured with the Decoration of honour of the province of Tyrol und das Decoration of Honour in Gold of the Republic of Austria. A hard-to-find bronze plaque at the former Gestapo headquarters in Herrengasse commemorates Robert Moser, who was tortured to death. There is a small information plaque at the house at Anichstraße 19, where Mayer was housed during his stay in Innsbruck. This perhaps most impressive episode in Innsbruck's city history only became known to a wider audience with the publication of the gripping book "Codename Brooklyn" by Peter Pirker, which received a great deal of international attention. However, Innsbruck perhaps owes its most enduring legacy to Wijnberg's radio messages: the code name for the city was after the New York neighbourhood where Mayer and he spent a long time, Brooklyn. Innsbruck was reborn after National Socialism as Innsbrooklyn.