The Operation Rheinubung began on 18 May, 1941. The new and powerful battleship of Deutsche Kriegsmarine Bismarck and her companion, the heavy cruiser Prinz Eugen sailed to the North Atlantic under the command of Admiral Gunther Lutjens. The ships sailed from the occupied Polish port of Gdynia (Gotenhafen) steaming to Bergen, Norwegian waters. After leaving the German waters, they continued on their route, heading north-west in order to pass into the Atlantic Ocean via the narrow Denmark Strait, between Greenland and Iceland instead of Iceland-Faeroes way. Admiral Lutjens was aware of the great risk of Royal Navy forces, so his decision was trying to reach the Atlantic via the Denmark Strait.
The plans of Rheinubung were secretly put into operation, but their secretive dash for the high seas didn't go unnoticed as they hoped. During the enroute to Bergen, a Swedish cruiser (Gotland) spotted the German forces and sent a message to the British relay in Stockolm. Also a resistance member of occupied Norway radioed a secret message to London too. By these reports, a British Spitfire reconnaissance plane spotted the two warships anchored in a Norwegian fjord (Grimstad) on 21. May. Actually, the two German ships arrived to Grimstad fjord, where their camouflage paints were removed and the fuel tanks of Prinz Eugen were refueled. But, Admiral Lutjens didn't want to refuel the Bismarck's tanks from the tanker Wollen. This decision was a mistake which proved to be an essential error for future operation of Bismarck. The reason of this decision can not be found today, but it is possible that the Admiral wanted to profit by the bad weather, which was good for Germans to hide themselves under the fog and clouds. So he wanted to get passed Iceland as soon as possible, and refuel the tanks when they reached open seas.
The British Home Fleet, however remained uncertain which route the battleship and the heavy cruiser would take on its way into the Atlantic waters. They could steam through the English Channel, between the coast of Scotland and the Faeroes Islands, between the Faeroes and Iceland, or through the Denmark Strait as the third option. Until the intended route became certainly clear, the Royal Navy couldn't risk concentrating the power along any of the possible routes.
The Home Fleet was under the command of Admiral Sir John Tovey, an intelligent and calm commander, who tried to balance his forces in a possible logical way. He was commanding the fleet from his flagship HMS King George-V. The two British heavy cruisers HMS Norfolk and HMS Suffolk were already patrolling the seas between Greenland and Iceland under the directives of Admiral Tovey. Sir Tovey also ordered battlecruiser HMS Hood, new battleship HMS Prince of Wales and six destroyers to assist the two cruisers and wait in alarm condition. The two warships were very powerful, who had a total of 18 heavy guns with 356mm. and 380mm. calibres.
HMS Hood was the pride of Royal Navy, flagship and the biggest battlecruiser of the world. She had 8 X 380mm. heavy guns, a top speed of 30.5 knots which was as good as the Bismarck's top speed and 42.000 tons of weight. But Hood was not a battleship instead she was a battlecruiser. Just because of this design factor, her armour was thinner than Bismarck (10 cm. citadel armour and 20 cm. hull belt armor). Although her guns were as powerful as Bismarck's L - 47 38 cm. guns, hurling aprroximately a total of 7000 Kg. full board salvo fire. Each shell weighed 870 Kg. and the maximum gunfire range was 33.500 yards. Hood was the pride of Royal Navy and she was a wonderful ship. But her armour was weak and the quality of armour plates were drastically lower than the Bismarck's chromium - nickel alloyed strong armor. Her commander was Admiral Sir Lancelot Holland, who was a very experienced and brave chief.
The other ship, Prince of Wales was a powerful battleship with a thick armour and 10 X 356mm. guns, but was a new and unexperienced one who was sailing with some civilian shipyard technicians on board. During the first two days, heavy fog cancelled the reconnaissance efforts on both sides. While British planes struggled to ascertain whether the German ships had left Norway, German scout planes incorrectly reported that all the ships of Admiral Tovey (the Home Fleet) still lay at anchor at Scapa Flow.