Just a few of them...
During any conflict, especially in one with the scope of the Second World War, it takes constant changes in battle tactics to beat the enemy. There are far more than we could ever cover, so we will primarily discuss three of them, due to how much they influenced the overall war, for both sides. Two will be strategies used by Germany in the Atlantic Ocean and Europe, while the third will cover an American battle plan in the Pacific Theater of War. All three will fill in gaps that we did not cover while discussing the major battles of the war itself.
Battle of the Atlantic (September 3, 1939-May 8, 1945)
The Battle of the Atlantic was the longest running engagement of the entire conflict. It began on the third day of the war, and lasted until V-E Day. The term "battle" is used loosely here, considering it was hundreds of battles all part of one overall plan. Considering Great Britain was an island, the English relied on a constant stream of ships to bring in supplies like food, medicine, oil, and other materials, in order to keep the country running every month. The Germans knew that the only way to defeat England, or at least choke them into leaving the war altogether, was to cut off this precious supply line.
The Kriegsmarine (German Navy), built dozens of U-Boats and continually sent them deep into the Atlantic Ocean to destroy any Allied supply ship they could find. Even after creating naval convoys which were escorted by warships, there was a "Mid-Atlantic Gap" where Allied air cover could not provide assistance, so the U-Boats could sink ships almost at will in this area. They became even deadlier when the Germans began using "Wolf-pack" tactics, in which multiple submarines would gang up on a single supply convoy.
Eventually though, British Intelligence broke the German Enigma code, and could read their messages. After this, Allied ships were able to know where the U-Boats would attack, and prepare for them accordingly. After Pearl Harbor especially, and the addition of the U.S. Navy to maritime patrols, the U-Boats came under heavier fire, despite still having the upper hand. Importantly, technology finally caught up with the Germans. The Allies had created sonar, which sent out sound waves into the water, and if they bounced back to the ship sending them, they could locate the exact place of a submarine. Furthermore, better depth charges cut up the U-Boats, and new American aircraft carriers being built eventually closed the Mid-Atlantic Gap.
The Germans did do considerable damage though, and sunk over 3,500 Allied supply and warships during the course of the war. The last few ships to be sunk were actually only hours before the official German surrender on V-E Day. The main point here is that the overall battle plan to choke Britain into submission simply did not work, and this made the Allied landings in North Africa, Italy, and on D-Day actually possible. U-Boats did serve a vital purpose, and almost cut off England's supply line, but the Germans simply could not build and supply the amount of them necessary to sink enough ships at a fast enough rate. Adequate supplies were able to reach England every month, and the addition of United States manufacturing power made it so that the Allies could build supply ships faster than the Germans could sink them.
The Kriegsmarine (German Navy), built dozens of U-Boats and continually sent them deep into the Atlantic Ocean to destroy any Allied supply ship they could find. Even after creating naval convoys which were escorted by warships, there was a "Mid-Atlantic Gap" where Allied air cover could not provide assistance, so the U-Boats could sink ships almost at will in this area. They became even deadlier when the Germans began using "Wolf-pack" tactics, in which multiple submarines would gang up on a single supply convoy.
Eventually though, British Intelligence broke the German Enigma code, and could read their messages. After this, Allied ships were able to know where the U-Boats would attack, and prepare for them accordingly. After Pearl Harbor especially, and the addition of the U.S. Navy to maritime patrols, the U-Boats came under heavier fire, despite still having the upper hand. Importantly, technology finally caught up with the Germans. The Allies had created sonar, which sent out sound waves into the water, and if they bounced back to the ship sending them, they could locate the exact place of a submarine. Furthermore, better depth charges cut up the U-Boats, and new American aircraft carriers being built eventually closed the Mid-Atlantic Gap.
The Germans did do considerable damage though, and sunk over 3,500 Allied supply and warships during the course of the war. The last few ships to be sunk were actually only hours before the official German surrender on V-E Day. The main point here is that the overall battle plan to choke Britain into submission simply did not work, and this made the Allied landings in North Africa, Italy, and on D-Day actually possible. U-Boats did serve a vital purpose, and almost cut off England's supply line, but the Germans simply could not build and supply the amount of them necessary to sink enough ships at a fast enough rate. Adequate supplies were able to reach England every month, and the addition of United States manufacturing power made it so that the Allies could build supply ships faster than the Germans could sink them.
Blitzkrieg (Invented in the 1930's, and used during World War II in German battle plans)
You were probably wondering at some point while learning about the battles during World War II, how the Germans were able to take so much land so quickly, (from France to the gates of Moscow) considering how slow the previous conflict had been, which is an excellent question.
The idea for Blitzkrieg, or "Lightning War," was thought of in the modern sense after World War I. The enormous deadlock of trench warfare was something that no country really wanted to repeat, and people went about finding ways of creating more mobile armies again. A number of military tacticians, including the well-known Heinz Guderian, saw the potential in tanks coordinated with foot soldiers and planes, and how they could quickly defeat a line of enemy trenches if used together correctly.
Basically, blitzkrieg was designed to confuse enemy forces by having a highly mobile army with concentrated firepower on one specific location. The aim is to create a short military engagement which reduces casualties and the needed amount of ammunition. The German used it to some success during the invasion of Poland in 1939, but used it most effectively when invading France, Belgium, and the Netherlands in 1940. It was also used to greatest effect during the North African Campaign, the invasion of the Soviet Union, the Battle of the Bulge, and was ironically adapted and used by U.S. General George Patton against Germany itself.
Though highly effective when used offensively of course, blitzkrieg is useless in a defensive war, which is what Germany was mainly fighting after 1942. Importantly, it relied on complete cooperation between the army, navy, and air forces, which seldom happened, because all three were constantly competing for the same limited resources Germany had available to them. On top of that, the army could barely work among themselves to begin with, much less anyone else. Throughout the war, the Wehrmacht’s (German Army) failure to work together was one of Germany's biggest weaknesses, and shows that Germany's military was not the smooth machine that most people say it was.
Blitzkrieg is still a concept used today, and it is incorporated into many modern battle plans. It continues to work well with the constant advances in communication between different units within a military force, which is what made it originally useful with the installation of radios into German tanks.
The idea for Blitzkrieg, or "Lightning War," was thought of in the modern sense after World War I. The enormous deadlock of trench warfare was something that no country really wanted to repeat, and people went about finding ways of creating more mobile armies again. A number of military tacticians, including the well-known Heinz Guderian, saw the potential in tanks coordinated with foot soldiers and planes, and how they could quickly defeat a line of enemy trenches if used together correctly.
Basically, blitzkrieg was designed to confuse enemy forces by having a highly mobile army with concentrated firepower on one specific location. The aim is to create a short military engagement which reduces casualties and the needed amount of ammunition. The German used it to some success during the invasion of Poland in 1939, but used it most effectively when invading France, Belgium, and the Netherlands in 1940. It was also used to greatest effect during the North African Campaign, the invasion of the Soviet Union, the Battle of the Bulge, and was ironically adapted and used by U.S. General George Patton against Germany itself.
Though highly effective when used offensively of course, blitzkrieg is useless in a defensive war, which is what Germany was mainly fighting after 1942. Importantly, it relied on complete cooperation between the army, navy, and air forces, which seldom happened, because all three were constantly competing for the same limited resources Germany had available to them. On top of that, the army could barely work among themselves to begin with, much less anyone else. Throughout the war, the Wehrmacht’s (German Army) failure to work together was one of Germany's biggest weaknesses, and shows that Germany's military was not the smooth machine that most people say it was.
Blitzkrieg is still a concept used today, and it is incorporated into many modern battle plans. It continues to work well with the constant advances in communication between different units within a military force, which is what made it originally useful with the installation of radios into German tanks.
Island Hopping (1942-1945)
The battle plan of Island Hopping is actually pretty much exactly as it sounds: American forces would go from one island to another in the Pacific. There is one important thing to understand here though, despite its simplicity, the point of it was to reduce casualties, and get American forces as close to Japan, as quickly as possible.
If you look at the map above, you will see the path the American Army, Navy, and Marines took to get to Japan. A middle route through the Pacific, and a lower route through the southern and western Pacific. There are thousands of islands which cover the Pacific Ocean, and attacking every single one of them which the Japanese controlled would take many years, and would inflict enormous casualties on landing forces. This is why (again, looking at the map), you see there are only a handful of major battle sites in the Pacific Theater.
This is where the "hopping" comes in. Instead of attacking every island, the United States took specific islands in larger chains, and then moved on. They would build airfields on the islands they took, and use planes to isolate the other islands which still had Japanese troops on them by destroying their supplies and ability to be a fighting force. These islands also served as bases needed to resupply ships while they traveled back and forth between battlefields.
As you can see, the plan was extremely successful, and allowed the U.S. to advance to islands in air range of Japan within two years, and within physical striking distance within three years after the fall of Iwo Jima and Okinawa. In fact, the planes which dropped the atomic bombs on Hiroshima and Nagasaki came from the airfields in the Marianas Islands, which were taken by U.S. forces in the middle of 1944.
If you look at the map above, you will see the path the American Army, Navy, and Marines took to get to Japan. A middle route through the Pacific, and a lower route through the southern and western Pacific. There are thousands of islands which cover the Pacific Ocean, and attacking every single one of them which the Japanese controlled would take many years, and would inflict enormous casualties on landing forces. This is why (again, looking at the map), you see there are only a handful of major battle sites in the Pacific Theater.
This is where the "hopping" comes in. Instead of attacking every island, the United States took specific islands in larger chains, and then moved on. They would build airfields on the islands they took, and use planes to isolate the other islands which still had Japanese troops on them by destroying their supplies and ability to be a fighting force. These islands also served as bases needed to resupply ships while they traveled back and forth between battlefields.
As you can see, the plan was extremely successful, and allowed the U.S. to advance to islands in air range of Japan within two years, and within physical striking distance within three years after the fall of Iwo Jima and Okinawa. In fact, the planes which dropped the atomic bombs on Hiroshima and Nagasaki came from the airfields in the Marianas Islands, which were taken by U.S. forces in the middle of 1944.