Denmark and Norway were invaded by Germany on 9 April 1940. A number of Norwegians were immediately arrested, and two months later the occupying force established the first prisoners' camp at Ulven Ulven Camp (), outside Bergen. As tensions intensified between the Nazi authorities and the resistance, increasing numbers of Norwegians were arrested and detained in Norwegian prisons and camps; and later deported to camps in Germany. The first groups of Norwegian prisoners arrived in Sachsenhausen camp in early 1940.