The signatories of the Washington Naval Treaty were the U.S., UK, Japan,
France, and Italy; at that time the only nations in the world with significant battlefleets. As a result, the terms of the treaty, and the subsequent treaties of
London 1930 and
London 1936, had a decisive effect on the future of capital ship design. The treaties extended the definition of
capital ship to cover all warships exceeding 10,000 tons
standard displacement or carrying guns exceeding
calibre; imposed limits on the total tonnage of capital ships allowed to each signatory; and fixed an upper limit of standard displacement for all future construction. These restrictions effectively signaled the end of the battlecruiser as a distinct category of warship, since any future big-gun cruiser would count against the capital ship tonnage allowance. It also greatly complicated the problem of fast battleship design, since the 35,000-ton limit closed off the most direct route to higher speed, as the increasing length-to-beam ratio would have meant a bigger ship; it required the development of more compact and powerful propulsion plants and lighter high-strength armour plating over the next two decades to make fast battleships feasible within the displacement limit. in 1940 Evidence of continued interest in high-speed capital ships is given by the fact that, although the signatories of the treaties were allowed to build gunned ships as their existing tonnage became due for replacement, most of them passed up the opportunity to do so, preferring instead lighter-armed but faster ships. A British Admiralty paper of 1935 concludes that a balanced design with speed and 16-inch guns would not be possible within the 35,000 ton limit, since it would be either insufficiently armoured or too slow; it is clear that by this date the speed of the
Nelsons was considered insufficient. The recommended design (never built) was one with nine guns and speed "not less than "; the that was actually built was similar to the recommended design but mounted ten guns (down from twelve guns of the initial design due to top weight concerns) in an effort to convince other naval powers to abide by the 14-inch calibre limit of the Second London Treaty. Although the calibre "escalator clause" increasing the limit back to 16 inches was invoked in April 1937 due to Italy's and Japan's refusal to sign the treaty, the British chose to proceed with the 14-inch guns on the
King George V since they needed these battleships badly, as redesigning the ships for 15-inch guns would be an eighteen month delay, and instead incorporated larger guns in follow-on designs. The 15-inch-gunned and es, built in the 1930s by Italy and France respectively, reflect similar priorities to the British. Under the terms of the
Anglo-German Naval Agreement of 1935 that effectively made Germany a party to the Second London Treaty, the German was built as a response to the
Richelieu class and also mounted 15-inch guns, although the ships were secretly considerably larger than the limits of the treaties. In 1937, the
Soviet Union signed the Anglo-Soviet Quantitative Naval Agreement and also agreed to abide by the terms of the Second London Treaty when beginning to design their
Project 23 or (never completed due to the eventual
German invasion), although they added a proviso that allowed them to build ships of unlimited size to face the Japanese navy if they notified the British. Four capital ships of the treaty era were built to displacements appreciably less than the 35,000-ton limit; the French and and the German and . The was built in response to the German '
(or "pocket battleship") . The ' were, in effect, a revival of the late 19th century concept of the commerce-raiding
armoured cruiser; long-ranged, heavily armed, and fast enough to evade a conventional capital ship. Likewise, the
Dunkerque, can be regarded as a revival of the armoured cruiser's nemesis, the battlecruiser. With 29-knot speed and 330 mm (13 inch) guns, she could operate independently of the fleet, relying on her speed to avoid confrontation with a more powerful adversary, and could easily overtake and overwhelm a
Panzerschiff, just as
Sturdee's battlecruisers had done to
von Spee's cruisers at the
Falkland Islands in 1914. On the other hand, as a member of the line of battle, alongside the elderly and slow dreadnoughts that made up the rest of the French battlefleet, the design would make no sense, since her speed would lose its value and neither her armament nor her protection would be at all effective against a modern 16-inch gunned battleship such as
Nelson. The
Scharnhorst and
Gneisenau were Germany's response to the
Dunkerques. They were an attempt to redress the inadequacies of the '
design in speed, survivability and powerplant (the diesel engines of the ' were unreliable and produced severe vibration at high speed), and used much material assembled for the '
programme (most significantly, the six triple gun mountings originally intended for ' D to F). Although much larger than the
Dunkerques, the
Scharnhorsts were also not intended for the line of battle; apart from their insufficient armament, set-piece battles against the vastly more numerous Allied battlefleets had no place in Germany's strategic requirements. Instead, the two German ships relied throughout their career on their superlative speed (over 32 knots) to evade the attentions of Allied capital ships. On
Gneisenau, the nine
28.3 cm SK C/34 guns in three triple turrets were supposed to be replaced with six
38.1 cm SK C/34 guns in twin turrets, which would have rectified her key weakness (although six main naval guns was fewer than WWI-era or Treaty battleships with eight or nine main guns), but work was cancelled in 1943 due to changing wartime conditions. The
Scharnhorst-class have been occasionally referred in English language reference works as battlecruisers due to their weak armament while having high speed and being well-protected, similar to how
Kaiserliche Marine battlecruisers were designed. The treaties also allowed the reconstruction of surviving battleships from the First World War, including up to additional protection against torpedoes, high-altitude bombing and long-range gunnery. In the late 1930s, the Italian and Japanese navies opted for extremely radical reconstructions: in addition to replacing the powerplant in their existing ships, they lengthened the ships by adding extra sections amidships or aft. This had a double benefit; the extra space allowed the size of the powerplant to be increased, while the extra length improved the speed/length ratio and so reduced the resistance of the hull. As a result, both navies realised significant increases in speed; for example the Japanese was increased from 23 to , and the Italian from . France, the UK and the US took a less radical approach, rebuilding their ships within their original hulls; boilers were converted to oil-firing or replaced, as were the engines in some cases, but increases in the output of the powerplant were generally canceled out by increases in the weight of armour, anti-aircraft armament and other equipment. s in the
Bungo Channel, 20 October 1941 The exception to the European battleship trend was Japan, which refused to sign the Second London Treaty. It rather uncharacteristically settled for a moderate speed of 27 knots, for the sake of exceptionally high levels of protection and firepower in the -gunned, displacement . Furthermore, although the Soviet Union was nominally held to the 35,000-ton Second London Treaty limit by signing the Anglo-Soviet Quantitative Naval Agreement of 1937, it only paid lip service to the agreement and the
Sovetsky Soyuz design, with nine 16-inch guns and 28-knot speed, quickly grew to over when laid down in 1938, although the eventual German invasion would prevent their completion. After much debate, the US settled on two 35,000 ton classes, also with a speed of 27 knots, in the and es. Due to treaty restrictions, firepower and protection were emphasised first, although both did manage respectable speed increases compared to their World War I contemporaries to be able to operate as carrier escorts. The US signed the Second London Treaty but was quick to invoke the "escalator clause" to increase the main battleship caliber from 14 to 16 inches as Italy and Japan refused to adopt it. This made the
North Carolinas somewhat unbalanced ships, with armor being designed to resist shells from the 14-inch guns that it was originally intended to carry, but being up-gunned during construction. The
South Dakotas rectified this with protection proof against 16-inch guns, albeit the original lightweight shells but not the super-heavy shells they were ending up equipped with. In order to counter the increase in armor weight and stay within tonnage limits, the
South Dakota class had to go with a shorter hull to reduce the length of the required protected area, compensating by installing more powerful machinery than in the
North Carolinas, and this made the ships somewhat cramped in terms of accommodation. The balanced 35,000-ton design was achieved by combining highly efficient lightweight double-reduction gear machinery with high pressure turbines, which reduced the length and volume of the armored citadel, while a sloped internal armored belt increased protection without increasing overall armor thickness although this had the disadvantage of being more difficult to repair (the hull had to be cut out first to replace the damaged armor). With Japan's withdrawal from the Second London Treaty and refusal to disclose any details about their battleship construction, the remaining signatories of UK, US, and France invoked the treaty's tonnage "escalator clause" in March 1938 that increased standard displacement limit from 35,000 tons to . Under the new limit, the UK and the US ordered the 16-inch-gunned and respectively in 1939, while the French began designing the . Despite the new limit, the UK chose to design the
Lion-class to due to limits of docking infrastructure (particularly the major naval installations at
Rosyth and
Portsmouth) and costs; the French would limit the
Alsace-class to that tonnage for similar logistical reasons. The , 45,000-ton
Iowa-class was intended serve as the fast division of the battle line or be detached to intercept fast capital ships such as the . With the additional tonnage, the
Iowas had new 16-inch guns with a greater maximum range, and they had even more powerful engines and a lengthened hull for a significantly faster speed over the
North Carolinas and
South Dakotas. ==World War II designs==