The most critical problem for the Western Allies in the northern European theater in 1941–42 was the urgent need to secure the war matériel being sent to the Soviet Union. Initially, the Germans did not react strongly against the Allied convoys sailing to northern Russia. However, that began to change quickly after February 1942, when the Germans redeployed almost all their heavy surface forces and a large number of U-boats from home waters to northern Norway.
"The Destruction of Convoy PQ17: 27 June–10 July 1942,"
Naval War College Review: Vol. 69
, Article 7.
Available at: http://digital-commons.usnwc.edu/nwc-review/vol69/iss3/7