UN-Restricted sub warfare was conducted in both WWI & WWII, & in all oceans & by all major combatants (US, Japan, Germany, Britain, Russia, etc.): Atlantic Ocean, Pacific Ocean, Indian ocean, etc.
Un-restricted sub warfare simply meant, "not giving prior warning to the vessel to be sunk."
Ground warfare occurred in the Republic of SOUTH Vietnam. Riverine warfare occurred in the Republic of SOUTH Vietnam; and some sea warfare occurred along the coastline of NORTH Vietnam. The Air War occurred in the skies of NORTH Vietnam. The Covert War occurred in Laos and Cambodia.
In almost all navies during WWII, submarines were called "boats"...not ships. In Germany, a sub was called an "under-sea-boat", with German enthusiasts preferring to use the German spelling/pronunciation of "boot" for boat. Bottom line: "U-Boat submarine warfare..." is simply submarine warfare (Sub Warfare is what most boys in America grew up calling it).
US subs sank Japanese vessels in the Pacific & German subs sank British/US vessels in the Atlantic. It might be a toss up, if it weren't for the fact that: 1. That's all the DKM did was sub warfare (after the US got into the war). 2. The USN was doing alot more than sub warfare; they were fighting cruiser, destroyer, PT Boat, Battleship, and Aircraft Carrier warfare. Consequently, US subs did alot of sinking...but were alot smaller sub force than the DKM (German Navy). The Atlantic most likely saw more unrestricted sub warfare. The Atlantic was a "Guerre de Course," a war of commerce raiding-NOT a battle between surface warships.
Most likely Germany, they had the U-boats (underwater boats)
Germany attempting to from a military alliance with Mexico
The United States declared war on Germany (then, later, Austrian-Hungary) in 1917 for several reasons. The most important was Germany's unrestricted submarine warfare campaign, which led to the loss of American goods and lives despite America's neutral status. Also involved was growing American sympathy for the Allied cause and growing distrust of Germany, in particular.
Diesel-Electric submarines had been in use for years before WWI; there were many commissioned boats on both sides of the war before it even started, so there is no one particular submarine used in WWI. However, the one boat that arguably had the most impact at the beginning of WWI was the German submarine U-20, which was responsible for sinking the British passenger liner RMS Lusitania. This single submarine attack had repercussions throughout the war, causing policy shifts in Germany considering submarine warfare, and which affected submarine warfare in general forever.
The flow of war material to France by way of Britain was preventing the German army from gaining an advantage in the fighting. Germany hoped to stop the resupply of the Allies, especially from the United States, and also to deter and demoralize the British, who were resisting them the most successfully in France. The Germans also hoped to relax the British naval blockade of German shipping.
Weapons such as poison gas were developed as a reaction to the stalemate that occurred throughout most of the war.
The US entered into WWI as the result of unrestricted German submarine attacks on civilian cruise liners. The most famous of these is when a German U-Boat attacked and sunk the luxury liner the Luisitania (sister ship of the Titanic) killing hundreds of American, British, and Irish civilians. Furthermore, the interception and decoding of the Zimmerman Telegram by British intelligence revealed to the US a German plot to subdue American forces by bringing Mexico into the war as well, thus allowing the resumption of unrestricted submarine warfare. As a result of these aggressive maneuvers, the US government decided to change their stance from neutral, soon after declared war on Germany and the other cental powers.
German submarine attacks on merchant ships
For most of the war they were neutral, they saw it as an European war that they had no part in. However over time more and more people in the US realized that what happens in other countries doe s affect them, particularly when ever increasing numbers of American were being killed by Germany's unrestricted submarine warfare program. Eventually the American public sentiment turned toward entering the war on the Allied side.