Capital ship

The capital ships of a navy are its most important warships; they are generally the larger ships when compared to other warships in their respective fleet. A capital ship is generally a leading or a primary ship in a naval fleet.

There is usually no formal criterion for the classification, but it is a useful concept in naval strategy; for example, it permits comparisons between relative naval strengths in a theatre of operations without the need for considering specific details of tonnage or gun calibres.

A notable example of this is the Mahanian doctrine, which was applied in the planning of the defence of Singapore in World War II, where the Royal Navy had to decide the allocation of its battleships and battlecruisers between the Atlantic and Pacific theatres. The Mahanian doctrine was also applied by the Imperial Japanese Navy, leading to its preventive move to attack Pearl Harbor and the capital ships of the U.S. Pacific Fleet. The naval nature of the Pacific Theater of Operations, more commonly referred to as the Pacific War, necessitated the United States Navy mostly deploying its battleships and aircraft carriers in the Pacific. The war in Europe was primarily a land war; consequently, Germany's surface fleet was small, and the Allied escort ships needed in the Battle of the Atlantic were mostly destroyers and destroyer escorts to counter the U-boat threat.

Before the advent of the all-steel navy in the late 19th century, a capital ship during the Age of Sail was generally understood as a ship that conformed to the Royal Navy's rating system of a ship of the line as being of the first, second, third or fourth rates:

Frigates were ships of the fifth rate; sixth rates comprised small frigates and corvettes. Towards the end of the Napoleonic Wars and into the late 19th century, some larger and more powerful frigates were classified as fourth rates.

The term "capital ship" was coined in 1909 and formally defined in the limitation treaties of the 1920s and 1930s, in the 1922 Washington Naval Treaty, 1930 London Naval Treaty, and 1936 Second London Naval Treaty. This applied mainly to ships resulting from the dreadnought revolution; dreadnought battleships (also known first as dreadnoughts and later as battleships) and battlecruisers. The term is also defined in the 1936 Montreux Convention.

In the 20th century, especially in World Wars I and II, typical capital ships would be battleships and battlecruisers. All of the above ships were close to 20,000 tons displacement or heavier, with large caliber guns and heavy armor protection.

Cruisers, despite being important ships, were not considered capital ships. An exception to the above in World War II was the Deutschland-class cruiser. Though this class was technically similar to a heavy cruiser, albeit slower but with considerably heavier guns, they were regarded by some as capital ships (hence the British label "Pocket battleship") since they were one of the few heavy surface units of the Kriegsmarine. The American Alaska-class cruiser, Dutch Design 1047 battlecruiser and the Japanese Design B-65 cruiser, planned specifically to counter the heavy cruisers being built by their naval rivals, have been described as "super cruisers", "large cruisers" or even "unrestricted cruisers", with some advocating that they even be considered battlecruisers; however, they were never classified as capital ships.