By the middle of April, the Germans started to advance out of Oslo to break the somewhat over-ambitiously named "iron ring" around the capital.
General Hvinden Haug has been criticised by historians for without a fight abandoning prepared and supposedly easily defensible positions along the river Nitelva at Lillestrøm thereby giving up the mustering places and remaining stores of the Norwegian Army at Gardermoen.
[2] Norwegian troops managed to halt the Germans temporarily in Hakadalen, at Bjørgeseter and at Strandlykkja by Lake Mjøsa blocking two out of three main routes out of Oslo going north.
The Norwegians were pushed back northwards, desperately trying to slow the German advance, and waiting for allied reinforcements.
The first British units arrived at Lillehammer around 21 April, but this proved too late, for on the same day the Germans decisively defeated the Norwegians at the battles of Lundehøgda and Bråstad and achieved a breakthrough to the Gudbrandsdalen, the heartland of Eastern Norway.