In the North Atlantic, German U-boats attempted to cut supply lines to the United Kingdom by sinking merchant ships. In the first four months of the war they sank more than 110 vessels. In addition to supply ships, the U-boats occasionally attacked British and Canadian warships. One U-boat sank the British carrier HMS Courageous, while another managed to sink the battleship HMS Royal Oak in her home anchorage of Scapa Flow.
In the summer of 1941, the Soviet Union entered the war on the side of the Allies. Although the Soviets had tremendous reserves in manpower, they had lost much of their equipment and manufacturing base in the first few weeks following the German invasion. The Western Allies attempted to remedy this by sending Arctic convoys, which travelled from the United Kingdom and the United States to the northern ports of the Soviet Union - Archangel and Murmansk. The treacherous route around the North Cape of Norway was the site of many battles as the Germans continually tried to disrupt the convoys using U-boats, bombers, and surface ships.
Following the entry of the United States into the war in December 1941, U-boats sank shipping along the East Coast of the United States and Canada, the waters around Newfoundland, the Caribbean Sea, and the Gulf of Mexico. They were initially so successful that this became known among U-boat crews as the second happy time. Eventually, the institution of shore blackouts and an interlocking convoy system resulted in a drop in attacks and U-boats shifted their operations back to the mid-Atlantic.
The turning point of the Battle of the Atlantic took place in early 1943 as the Allies refined their naval tactics, effectively making use of new technology to counter the U-Boats. The Allies produced ships faster than they were sunk, and lost fewer ships by adopting the convoy system. Improved anti-submarine warfare meant that the life expectancy of a typical U-boat crew would be measured in months. The vastly improved Type 21 U-boat appeared as the war was ending, but too late to affect the outcome. In December 1943, the last major sea battle between the Royal Navy and the German Navy took place. At the Battle of North Cape, the German battleship Scharnhorst, was sunk by HMS Duke of York, HMS Belfast, and several destroyers.
The Pacific War was the part of World War IIâ€”and preceding conflictsâ€”that took place in the Pacific Ocean, its islands, and in East Asia, between July 7, 1937, and August 14, 1945. The most decisive actions took place after the Empire of Japan attacked various countries, later known as the Allies (or Allied powers), on or after December 7, 1941, including an attack on United States forces at Pearl Harbor. The attack on Pearl Harbor (called the Hawaii Operation or Operation Z by the Japanese Imperial General Headquarters, and the Battle of Pearl Harbor by some Americans) was a surprise military strike conducted by the Imperial Japanese Navy against the United States naval base at Pearl Harbor, Hawaii on the morning of December 7, 1941. The attack was intended as a preventive action in order to keep the U.S. Pacific Fleet from influencing the war that the Empire of Japan was planning in Southeast Asia, against Britain and the Netherlands, as well as the U.S. in the Philippines. The base was attacked by 353 Japanese aircraft in two waves, launched from six aircraft carriers. The attack came as a profound shock to the American people and led directly to the United States entry into World War II in both the Pacific and European theaters. Despite numerous historical precedents for unannounced military action, the lack of any formal warning by Japan, particularly while negotiations were still apparently ongoing, led to President Franklin D. Roosevelt proclaiming December 7, 1941, "a date which will live in infamy".
Between 1942 and 1945, there were four main theatres in the Pacific War, corresponding with and defined by the major Allied commands in the war against Japan. U.S. sources refer to two theaters within the Pacific War: the Pacific Theater of Operations (PTO) and the China Burma India Theater (CBI). However these were not operational commands. In the PTO, the Allies divided operational control of their forces between two supreme commands, known as Pacific Ocean Areas and Southwest Pacific Area.
Island hopping was an important military strategy in the Pacific Theater of World War II. The strategy employed by the Allies of World War II Combined Chiefs of Staff, beginning with Operation Cartwheel, was to bypass heavily fortified Japanese positions and instead concentrate the limited Allied resources on strategically important islands that were not well defended but capable of supporting the drive to the main islands of Japan. This strategy was possible in part because the Allies used submarine and air attacks to blockade and isolate Japanese bases, weakening their garrisons and reducing the Japanese ability to resupply and reinforce. Thus troops on islands which had been bypassed, such as the major base at Rabaul, were useless to the Japanese war effort and left to "wither on the vine."
Hard-fought battles on the Japanese home islands of Iwo Jima, Okinawa, and others resulted in horrific casualties on both sides, but finally produced a Japanese retreat. Faced with the loss of most of their experienced pilots, the Japanese increased their use of kamikaze tactics in an attempt to create unacceptably high casualties for the Allies. After the turning point of the Pacific where a third of the Imperial Japanese Navy fleet was hit in the Battle of Midway, the United States Department of the Navy recommended various positions for and against an invasion of Japan in 1945. Some staff proposed to force a Japanese surrender through a total naval blockade or air raids.