Sweden occupied the undefended Norwegian province of Jemtland, which was quickly reconquered by a counterattack by forces under command of the Norwegian governor of Trøndelag. The forces were unwilling to launch a counterattack on Swedish land. In 1564 the Swedes marched under Claude Collart and re-occupied Jemtland, as well as Herjedalen and Trøndelag, including the city of Trondheim. Initially facing little opposition from the locals, their subsequent ill treatment of the Trøndelag natives, along with tax pressure, laid the groundwork for later resistance to Swedish invasion. Also, Trøndelag was assisted by the governor of Bergenhus, Erik Rosenkrantz, who forced 3500 local peasants to assist him and his 50 professional soldiers. The Swedes saw Bergenhus as their next target. Although the 400 Swedish soldiers were repelled from Trøndelag, Sweden continued to occupy Jämtland and Härjedalen. These provinces were later regained by Denmark-Norway following the peace process in 1570. Sweden also launched attacks towards Eastern Norway. In the south-east Sweden captured Båhus Fortress, but lost it in 1566. Another part of the army marched through the valley Østerdalen in 1567, captured Hamar and continued towards Oslo. They reached as far as the Skiensfjord, and torched Skien at one point. In Oslo, however, citizens torched the city before invaders could seize it. From the south-east, more Swedish forces were then sent to aid in capturing Akershus Fortress near Oslo. These forces torched Konghelle and Sarpsborg on their way. They were repelled from Oslo by local forces together with men belonging to Erik Rosenkrantz and the King of Denmark and Norway. The Swedes retreated in a north-eastern direction, torching Hamar on their way, destroying Hamar Cathedral and the bishop's fortified palace Hamarhus.
Answer this question: How many years did the events take place?
6