Anti-aircraft warfare
Further investigation revealed that US powders would not work in the pom-pom. Bureau of Ordnance was well aware of the Bofors 40 mm gun. The firm York Safe and Lock was negotiating with Bofors to attain the rights to the air-cooled version of the weapon. At the same time Henry Howard, an engineer, and businessman became aware of it and contacted RAMD W. R. Furlong, chief of the Bureau of Ordnance. He ordered the Bofors weapon system to be investigated. York Safe and Lock would be used as the contracting agent. The system had to be redesigned for both the English measurement system and mass production, as the original documents recommended hand fitting parts and drilling to shape. As early as 1928 the US Navy saw the need to replace the .50 caliber machine gun with something heavier. The 1.1"/75 (28 mm) Mark 1 was designed. Placed in quadruple mounts with a 500 rpm rate of fire it would have fit the requirements. However, the gun was suffering teething issues being prone to jamming. While this could have been solved the weight of the system was equal to that of the quad-mount Bofors 40 mm while lacking the range and power that the Bofors provided. The gun was relegated to smaller less vital ships by the end of the war. The 5"/38 naval gun rounded out the US Navy's AA suite. A dual purpose mount, it was used in both the surface and AA roles with great success. 

Mated with the Mark 37 director and the proximity fuse it could routinely knock drones out of the sky at ranges as far as 13,000 yards.A 3"/50 MK 22 semiautomatic dual gun was produced but not employed before the end of the war and therefore beyond the scope of this article. However early marks of the 3"/50 were employed in destroyer escorts and on merchant ships. 3″/50 caliber guns (Marks 10, 17, 18, and 20) first entered service in 1915 as a refit to USS Texas (BB-35), and were subsequently mounted on many types of ships as the need for anti-aircraft protection was recognized. During World War II, they were the primary gun armament on destroyer escorts, patrol frigates, submarine chasers, minesweepers, some fleet submarines, and other auxiliary vessels, and were used as a secondary dual-purpose battery on some other types of ships, including some older battleships. They also replaced the original low-angle 4"/50 caliber guns (Mark 9) on "flush-deck" Wickes and Clemson-class destroyers to provide better anti-aircraft protection. The gun was also used on specialist destroyer conversions; the "AVD" seaplane tender conversions received two guns; the "APD" high-speed transports, "DM" minelayers, and "DMS" minesweeper conversions received three guns, and those retaining destroyer classification received six.

The Germans developed massive reinforced-concrete blockhouses, some more than six stories high, which were known as Hochbunker 'high bunkers' or "Flaktürme" flak towers, on which they placed anti-aircraft artillery. Those in cities attacked by the Allied land forces became fortresses. Several in Berlin were some of the last buildings to fall to the Soviets during the Battle of Berlin in 1945. The British built structures such as the Maunsell Forts in the North Sea, the Thames Estuary and other tidal areas upon which they based guns. After the war most were left to rot. Some were outside territorial waters, and had a second life in the 1960s as platforms for pirate radio stations, while another became the base of a micronation, the Principality of Sealand.

Some nations started rocket research before World War II, including for anti-aircraft use. Further research started during the war. The first step was unguided missile systems like the British 2-inch RP and 3-inch, which was fired in large numbers from Z batteries, and were also fitted to warships. The firing of one of these devices during an air raid is suspected to have caused the Bethnal Green disaster in 1943. Facing the threat of Japanese Kamikaze attacks the British and US developed surface-to-air rockets like British Fairey Stooge or the American Lark as counter measures, but none of them were ready at the end of the war. The Germans missile research was the most advanced of the war as the Germans put considerable effort in the research and development of rocket systems for all purposes. Among them were several guided and unguided systems. Unguided systems involved the Fliegerfaust (literally "aircraft fist") rocket launcher as the first MANPADS. Guided systems were several sophisticated radio, wire, or radar guided missiles like the Wasserfall ('waterfall') rocket. Owing to the severe war situation for Germany all of those systems were only produced in small numbers and most of them were only used by training or trial units.

Another aspect of anti-aircraft defence was the use of barrage balloons to act as physical obstacle initially to bomber aircraft over cities and later for ground attack aircraft over the Normandy invasion fleets. The balloon, a simple blimp tethered to the ground, worked in two ways. Firstly, it and the steel cable were a danger to any aircraft that tried to fly among them. Secondly, to avoid the balloons, bombers had to fly at a higher altitude, which was more favourable for the guns. Barrage balloons were limited in application, and had minimal success at bringing down aircraft, being largely immobile and passive defences.
The Allies' most advanced technologies were showcased by the anti-aircraft defence against the German V-1 cruise missiles (V stands for Vergeltungswaffe, 'retaliation weapon'). The 419th and 601st anti-aircraft gun battalions of the US Army were first allocated to the Folkestone-Dover coast to defend London, and then moved to Belgium to become part of the "Antwerp X" project coordinated from the Le Grand Veneur in Keerbergen. With the liberation of Antwerp, the port city immediately became the highest priority target, and received the largest number of V-1 and V-2 missiles of any city. The smallest tactical unit of the operation was a gun battery consisting of four 90 mm guns firing shells equipped with a radio proximity fuse. Incoming targets were acquired and automatically tracked by SCR-584 radar,. Output from the gun-laying radar was fed to the M9 gun director, an electronic analogue computer to calculate the lead and elevation corrections for the guns. With the help of these three technologies, close to 90% of the V-1 missiles, on track to the defence zone around the port, were destroyed.

Post-war: Post-war analysis demonstrated that even with newest anti-aircraft systems employed by both sides, the vast majority of bombers reached their targets successfully, on the order of 90%. While these figures were undesirable during the war, the advent of the nuclear bomb considerably altered the acceptability of even a single bomber reaching its target.
The developments during World War II continued for a short time into the post-war period as well. In particular the US Army set up a huge air defence network around its larger cities based on radar-guided 90 mm and 120 mm guns. US efforts continued into the 1950s with the 75 mm Skysweeper system, an almost fully automated system including the radar, computers, power, and auto-loading gun on a single powered platform. The Skysweeper replaced all smaller guns then in use in the Army, notably the 40 mm Bofors. By 1955, the US military deemed the 40 mm Bofors obsolete due to its reduced capability to shoot down jet powered aircraft, and turned to SAM development, with the Nike Ajax and the RSD-58. In Europe NATO's Allied Command Europe developed an integrated air defence system, NATO Air Defence Ground Environment (NADGE), that later became the NATO Integrated Air Defence System.
The introduction of the guided missile resulted in a significant shift in anti-aircraft strategy. Although Germany had been desperate to introduce anti-aircraft missile systems, none became operational during World War II. Following several years of post-war development, however, these systems began to mature into viable weapons. The US started an upgrade of their defences using the Nike Ajax missile, and soon the larger anti-aircraft guns disappeared. The same thing occurred in the USSR after the introduction of their SA-2 Guideline systems.

As this process continued, the missile found itself being used for more and more of the roles formerly filled by guns. First to go were the large weapons, replaced by equally large missile systems of much higher performance. Smaller missiles soon followed, eventually becoming small enough to be mounted on armoured cars and tank chassis. These started replacing, or at least supplanting, similar gun-based SPAAG systems in the 1960s, and by the 1990s had replaced almost all such systems in modern armies. Man-portable missiles, MANPADS, as they are known today, were introduced in the 1960s and have supplanted or replaced even the smallest guns in most advanced armies.
In the 1982 Falklands War, the Argentine armed forces deployed the newest west European weapons including the 35 mm Oerlikon GDF-002 twin cannon and Roland missile.