Aircraft carriers form the main capital ships of most modern-era blue-water navies.Battleships became the main form of capital ship after sailing vessels fell out of use, and remained so up to World War II. Shown is the German SMS Helgoland.Ships of the line (of battle) were the capital ships of the era of sail. Shown the Spanish Santa Ana, a very large example with 112 cannons.The capital ships of a navy are its most important warships; they generally possess the heaviest firepower and armor and are traditionally much larger than other naval vessels. 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 diameters.
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 their battleships and battlecruisers between the Atlantic and Pacific theatres. The Mahanian doctrine was also applied by the Imperial Japanese Navy, leading to their pre-emptive move to attack Pearl Harbor and the battleships of the U.S. Pacific Fleet. The United States Navy, on the other hand, deployed its battleships and aircraft carriers in the Pacific. Although the United States and the United Kingdom agreed upon a Germany-first grand strategy, Germany's surface fleet was small, and the escort ships used in the Second Battle of the Atlantic were mostly destroyers and destroyer escorts to counter the U-boat threat; only a few German capital ships, such as the Admiral Graf Spee, Scharnhorst and Bismarck were engaged by Allied ships.
Before the advent of the all-steel navy in the late 19th century, a capital ship was generally understood as a ship that conformed to the Royal Navy's classification of a ship of the line as being of the first, second, third or fourth rates:
- First rate: 100 or more guns, typically carried on three or four decks. Four-deckers suffered in rough seas, and the lowest deck could seldom fire except in calm conditions.
- Second rate: 90–98 guns.
- Third rate: 64 to 80 guns (although 64-gun third-raters were small and not very numerous in any era).
- Fourth rate: 46 to 60 guns. By 1756, these ships were acknowledged to be too weak to stand in the line of battle and were relegated to ancillary duties, although they also served in the shallow North Sea and American littorals where larger ships of the line could not sail.
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 definition of "capital ship" was formalized in the limitation treaties of the 1920s and 30s in the Washington Naval Treaty, London Naval Treaty, and 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.
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. Heavy 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 with considerably heavier guns, they were generally regarded as capital ships (hence the British label "Pocket battleship"). The Alaska-class cruisers, despite being oversized heavy cruisers and not true battleships/battlecruisers, were also considered by some to be capital ships.
During the Cold War, a Soviet Kirov-class large missile cruiser had a displacement great enough to rival WWII-era capital ships, perhaps defining a new battlecruiser for that era. However, others consider the Kirov to be just a supersized guided-missile cruiser.
It took until late 1942 before aircraft carriers were universally considered capital ships. The U.S. Navy was forced to rely primarily on their aircraft carriers after the attack on Pearl Harbor sank or damaged eight of their Pacific Fleet battleships.
In the 21st century, the aircraft carrier is the last remaining capital ship, with capability defined in decks available and aircraft per deck, rather than in guns and calibers. The United States possesses supremacy, in both categories of aircraft carriers, possessing not only 11 active duty supercarriers each capable of carrying and launching nearly 100 tactical aircraft, but an additional 12 amphibious assault ships as capable (in the "Sea Control Ship" configuration) as the light VSTOL carriers of other nations.
Ballistic missile submarines (or "boomers"), while important ships and similar in tonnage to early battleships, are usually counted as part of a nation's nuclear deterrent force and do not share the sea control mission of traditional capital ships. Many navies, including the Royal Navy and the United States Navy, consider these ships to be capital ships.
Some navies reserve specific names for their capital ships. Names reserved for capital ships include chiefs of state (e.g. Bismarck), important places (e.g. HMAS Australia), historically important naval officers or admiralty (e.g. De Ruyter), historical events (e.g. USS Constitution), traditional names (e.g. HMS Ark Royal). However there are some exceptions to the rule.
The term has been adopted into science fiction literature and culture to describe large spaceships used in military contexts, particularly where other naval terms have also been adopted in similar fashion; for example, sci-fi capital spaceships are often "carriers", that carry small fighters analogous to the way the real-world naval equivalent carries fighter aircraft, as well as functioning as "battleships".