sentences:  The supplies were vital in that they were needed to keep Russia in the war following her invasion by German forces in 1941.The PQ 17 convoy, which left Iceland at the end of June 1942, is known particularly for the tremendous losses among the merchant ships.The problems for convoy PQ 17 started early on the morning of 4 July 1942, with the appearance of a copper-yellow warhead of a torpedo, clearly visible just below the surface of the sea.