War on the sea
Before the war the national Navies were largely seen as the key to world dominance. The attempt by the German Empire to compete for sea dominance with the United Kingdom was one of the main reasons that brought to the beginning of the war.
The crown-jewels of every Navy that could afford them were the dreadnoughts, enourmous armored ships moved by steam turbines and armed exclusively with high caliber guns. The construction of these ships required considerable resources and their possession had a symbolic as well as a military value. The first dreadnought was launched by the British Empire in 1906. Already in 1911 more poweful and heavily armed ships, the "superdreadnoughts", were laid down by the world major navies. In 1914 the British Navy had 22 dreadnoughts, the German 15, the Italian 3.
Aside from the heavy battleships all the opposing fleets included mainly cruisers (ships with high caliber cannons but not heavily armored), light torpedo boats, destroyers (mainly used to protect slow and heavy vessels from smaller and faster ships) and submarines. In 1918 the British Navy launched the first aircraft carrier - an innovation that was forseen by many popularization magazines.
Despite the importance of the seas in military thought and in popular imagination, the first world war saw no decisive naval battle and victory was not achieved on the sea. The British fleet was mostly used to enforce a successful blockade on German ports. The only major battle between the British and German navies took place in Jutland in 1916 and had no clear winner, but its losses convinced the German admiralty to avoid future clashes and to put their hopes on submarine warfare.
The Central Powers made a large use of submarine warfare, in the attempt to contrast the more powerful British fleet and to thwart the commerce between the United States and the Allies. In 1915 and from 1917 Germany declared the seas around the British Isles to be a war zone, claiming the right to attack with no warning all the ships that crossed, civilian and neutral vessels included.
This "unrestricted submarine warfare" was a military success, but had grave consequences. As in the case of gas, flamethrowers and bombings on civilian objectives the fact that the submarines were used mostly by the Germans gave the Allies a powerful propaganda instrument and allowed them to accuse Germany of piracy and disregard for international law. It was indeed an "unconventional" form of war that, as aerial raids on urban centres, had civilian targets as its main objective.
In 1915 the public indignation over the sinking of the Lusitania, a passenger ship suspected to carry munitions to Britain, brought to a suspension of German attacks to neutral vessels. When the unrestricted attacks were resumed in 1917, due to the disastrous German economic situation, the United States declared war to Germany.
The first world war submarine spent most of its navigation time on the surface, diving under water only at the moment of the attack. Its speed was quite limited if confronted to the ships of the time, between 10 and 15 km/h under water. There was no other way to locate a submarine other than by sight: the sonar was experimented upon at the end of the conflict, too late to be used in real combat. The sighting of submarines were mostly made by airplanes and by observation balloons attached to the ships.
The fundamental elements of the submarine success were surprise, invisibility and the ability to disable the enemy ship in one single blow. For this reason the most effective responses to the submarine menace were mines anchored to the seabed (protecting stationary targets) and most of all the organization, from mid-1917, of ship convoys