The sea war in World War One took place mostly in the Atlantic Ocean and the North Sea. However important naval events also took place in the Mediterranean Sea and the Pacific Ocean.
The war at sea in World War One focused on trying to stop supplies from reaching the enemy while keeping your own supplies flowing and supporting your troops fighting on land.
The war at sea started as soon as war was declared in 1914. Some of the earliest fighting between the Allies and the Central Powers happened between opposing warships. The sea war lasted until the armistice in 1918 when Germany had to surrender her navy.
The main struggle at sea in World War One was between Britain and Germany. The two nations fought a handful of large sea battles but mostly the British fought against the German submarines - the U-Boats - that were sinking British supply ships.
However there were also naval battles in the Mediterranean between the Austro-Hungarian and Ottoman navies and the Allies, including France, the USA, Italy and Britain. The Mediterranean was important for the Allies because they needed to be able to get supplies to the Middle East and Gallipoli.
The role played by the submarine in World War One is key. Submarines were quite a new weapon in 1914 but Germany showed how deadly they could be. In World War Two the U-Boat would play an even greater part.