Given this early history, it is perhaps not surprising that it was only in Germany that development of anti-aircraft guns continued. In 1909 a number of Krupp's designs were shown, including adaptations of their 65 mm 9-pounder, a 75 mm 12-pounder, and even a 105 mm gun. By the start of World War I the 75 mm had become the standard German weapon, and came mounted on a large traverse that could be easily picked up on a wagon for movement.

Other countries involved seem to have largely ignored the possibility of aircraft being an important part of the hostilities, but this soon changed when German spotter aircraft started calling down increasingly accurate artillery fire. All armies soon deployed a number of guns based on their smaller field pieces, notably the French 75 mm and Russian 76.2 mm, typically simply propped up on some sort of embankment to get the muzzle pointed skyward. The British Army decided on an entirely new weapon, and deployed a 3 inch (76 mm) gun that was perhaps the best of the bunch.

In general these ad-hoc solutions proved largely useless. With little experience in the role, and no ability to spot the "fall" of their rounds with accuracy, gunners proved unable to get the altitude correct and most rounds fell well below their targets. The exception to this rule were the guns protecting spotting balloons, in which case the altitude could be accurately measured from the length of the cable holding the balloon. The Krupp guns were later supplied with an optical sighting system and soon improved their capabilities, but these sorts of systems were not deployed by other forces.

As aircraft started to be used in tactical roles against ground targets, these larger weapons proved too ponderous to aim at the quickly moving targets. Soon the forces were adding various machine gun based weapons mounted on poles. The British introduced a heavier weapon — their 1-pounder "pom-pom" (a 20 mm version of the Maxim Gun) on a elevated mounting. These short-range weapons proved more deadly, and the Red Baron arguably fell victim to an anti-aircraft Vickers machine gun.

When the war ended it was clear that the increasing capabilities of aircraft would require a much more serious attempt at downing them. Nevertheless the pattern had been set: anti-aircraft weapons would be based around heavy weapons attacking high-altitude targets and lighter weapons for use when they came to lower altitudes.

World War I had proven that the aircraft was an important part of the battlefield. As the capabilities of aircraft improved, and more specificially their engines, it was clear that their role in future combat would be even more critical as their warload grew. Many felt that the higher speeds and altitudes would render anti-aircraft systems useless, so little effort was put into improving systems.

Once again, it was only Germany that seriously considered what to do about this. They developed a number of new anti-aircraft weapons in the late 1920s and early 1930s, often in collaboration with Swiss and Swedish companies, including a new rapid-fire 20 mm gun for low-altitude work, and a 37 mm gun for low and medium altitudes. By the mid-1930s the 20 mm was considered to be too low power against the increasingly fast planes, but instead of introducing a new gun, Krupps managed to squeeze four of the existing 20 mm guns onto a single carriage of about the same weight. By the end of the war Germany had essentially given up on the 20 mm as lacking punch. It was never cleanly replaced however; the 37 mm was available in limited numbers, and a new dual-30 mm system based on the MK 103 aircraft gun was never put into widespread use.

Their high-altitude needs were originally going to be filled by a 75 mm gun from Krupp, designed in collaboration with their Swedish counterpart Bofors, but the specifications were later amended to require much higher performance. In response Krupp's engineers presented a new 88 mm gun design, the Flak 88. The eighty-eight would go on to become one of the most famous artillery pieces in history. First used in Spain during the Spanish Civil War, the gun proved to be one of the best anti-aircraft guns in the world, as well as particularly deadly against tanks. It is in this later role that it became most widely known, the bane of allied tank crews everywhere.

After the Dambusters raid in 1943 an entirely new system was developed that was required to knock down any low-flying aircraft with a single hit. The first attempt to produce such a system used a 50 mm gun, but this proved inaccurate and a new 55 mm gun replaced it. The system used a centralized control system including both search and targeting radar, which calculated the aim point for the guns after considering windage and ballisitics, the commands were then sent to the guns which used hydraulics to point themselves at high speeds. Operators simply fed the guns and selected the targets. This system, modern even by today's standards, was in late development when the war ended.

In the late 1920s the Swedish Navy had ordered the development of a 40 mm naval anti-aircraft gun from the Bofors company. The new gun proved to be light, fast and reliable, and a mobile version on a four wheel carriage was soon developed. Known simply as the 40 mm, it was adopted by some 17 different nations just before WWII and is still in use today in some applications such as on coast guard frigates.

At the start of the war Britain had started a slow upgrade to their own systems, including a new QF 3.75 inch (94 mm) gun in addition to their existing WWI-era 3 inch guns. Both were delivered with optical sighting systems for ranging. At the small-end of the scale a number of 20 mm designs were used, but testing showed, as Germany had discovered, that these weapons were of little use against modern aircraft.

They had already arranged license building of the 40 mm Bofors gun, and introduced these into service. These had the power to knock down aircraft of any size, yet were light enough to be mobile and easily swung. The gun became so important to the British war effort that they even produced a movie, The Gun, in order to make workers on the assembly line work harder. The Imperial measurement production drawings the British had developed were supplied to the Americans who produced their own (unlicensed) copy of the 40 mm at the start of the war - moving to licensed production in mid-1941.

Service trials demonstrated another problem however, that the problem of ranging and tracking the new high-speed targets was almost impossible – at shorter ranges the "lead" required (aiming in front of the target because it is moving) is so small that it can be done manually, and at very long ranges the apparent speed is so slow that existing manual calculators were good enough. For the ranges and speeds that the Bofors worked at neither solution was good enough.

The solution was automation, in the form of a mechanical computer, the Kerrison Predictor. Operators kept it pointed at the target, and the Predictor then calculated the proper aim point automatically and displayed it as a pointer mounted on the gun. The gun operators simply followed the pointer and loaded the shells. The Kerrison was fairly simple, but it pointed the way to future generations which incorporated radar for ranging, and then tracking.

Although they receive little attention, US Army anti-aircraft systems were actually quite good. Their smaller tactical needs were filled with quad-mounted 50 calibre (12.7 mm) machine guns, which were often mounted on the back of a half-track to form the Half Track GMC16, Anti-Aircraft. Although of even less power than Germany's 20 mm systems, they were at least widely available. Their larger 90 mm heavy guns would prove, as did the eighty-eight, to make an excellent anti-tank gun as well, and was widely used late in the war in this role. Finally just as the war was ending a new 120 mm gun with an impressive 60,000 ft altitude capability was introduced, the so-called stratosphere gun, which would continue in use after the war into the 1950s.

The Germans developed massive reinforced concrete blockhouses, some more than six stories high, which known as Hochbunker "High Bunkers" or "Flakturm" Flak Towers, on which they placed Anti-aircraft Artillery. Those which were 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 in the Thames estuary and other tidal areas on which they based Triple A. 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.

During the war the first ventures into the use of rocket-powered missiles for shooting down aircraft began. The British started with unguided missiles, the 2 inch RP, fired in large numbers from Z batteries. The firing of one of these devices during a an air raid is suspected to have caused the Bethnal Green Disaster in 1943. By the end of the war the British had developed a surface-to-air missile, Stooge, which would have been launched from Royal Navy ships against the Japanese Kamikaze attacks.

One 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 where a danger to any aircraft that tried to fly among them. Secondly, in avoiding the balloons, the bombers were forced up to a higher level which was more favourable for the guns. The barrage balloon was limited in application and direct success at bringing down aircraft - being largely immobile and passive weapons.

Post-war analysis demonstrated that even with newer anti-aircraft systems employed by both sides, the vast majority of bombers reached their targets successfully, on the order of 90%. This was bad enough during the war, but the introduction of the nuclear bomb into the equation upset things considerably. Now even a single bomber reaching the target would be generally unacceptable.

The developments during WWII 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. But given the general lack of success of guns against even propeller bombers, it was clear that any defence was going to have to rely almost entirely on interceptor aircraft. Despite this US efforts continued into the 1950s with the 75 mm Skysweeper system.

The US Airforce used the Semi Automatic Ground Environment computer network to track potential bomber attacks and command interceptor or missile attacks against them through the 1960s and 1970s.

Things changed with the introduction of the guided missile. Although Germany had been desperate to introduce them during the war, none were ready for service, and British countermeasures were likely to defeat them even if they were. With a few years of development however, these system started to mature into practical 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.

The evolution since this time has been a slow change from guns to missiles for the shorter range roles. Originally missiles were useful only as a replacement for the very largest of anti-aircraft guns, but by the 1960s they had been scaled down to the point where they were also replacing smaller weapons previously serviced by guns in the 40 mm to 57 mm range. Today man-portable missiles, known as MANPADS, are replacing even the very smallest of gun systems.

If current trends continue, missiles will replace gun systems completely in "first line" service. Guns are being increasingly pushed into specialist roles, such as the US Phalanx CIWS which uses a 20 mm M61 Vulcan gun firing at over 4,500 rounds per minute for last ditch anti-missile and anti-aircraft fighting. Even this formerly first-rate weapon is currently being replaced by a new missile system, the Rolling Airframe Missile, which is smaller, faster, and allows for mid-flight course correction (guidance) to ensure a hit.

Upsetting this development to all-missile systems is the current move to stealth-based aircraft. Long range missiles depend on long-range detection in order to provide significant lead. Stealth designs cut detection ranges so much that the aircraft is often never even seen, and when it is, often too late for an intercept. Systems for detection and tracking of stealthy aircraft are a major problem for anti-aircraft development.

Another potential weapon system for anti-aircraft use is the laser. Although originally intended to be used in this role since the late 1960s, the most modern laser systems are currently reaching what could be considered "experimental usefulness". In particular the Tactical High Energy Laser can be used in the anti-aircraft and anti-missile role. If current developments continue, it is reasonable to suggest that lasers will play a major role in air defence starting in the next 10 years.

Most western militaries integrate air defence purely with the traditional arms of the military (i.e. army, navy and air force). This is in contrast to some (largely communist or ex-communist) countries where not only are there provisions for air defence in the army, navy and air force but there are specific branches which deal only with the air defence of territory. For example, the Soviet PVO Strany. It should be noted that the USSR also had a separate strategic rocket force in charge of, among other things, nuclear ICBMs.

Virtually all modern vessels contain anti-aircraft weapon systems. Smaller boats and ships typically have machine-guns or fast cannons, which can often be deadly to low-flying aircraft if linked to a radar-directed fire-control system. Larger vessels (patrol boats, frigates, destroyers and on up) are typically equipped with surface-to-air missile systems, with increased range and deadliness as the vessel size increases. Some vessels like Aegis cruisers are as much a threat to aircraft as any surface-based air defence system. In general, naval vessels should be treated with respect by aircraft, however the reverse is equally true. Carrier battle groups are especially well defended, as not only do they typically consist of many vessels with heavy air defence armament but they are also able to launch fighter jets for combat air patrol overhead to intercept incoming airborne threats.

Some modern submarines are equipped with surface-to-air missile systems, since helicopters and anti-submarine warfare aircraft are significant threats.

Armies typically have air defence in depth, from integral MANPADS like Stinger and Igla at smaller force levels up to army-level missile defence systems such as Angara and Patriot. Often, the high-altitude long-range missile systems force aircraft to fly at low level, where anti-aircraft guns can bring them down. As well as the small and large systems, for effective air defence there must be intermediate systems. These may be deployed at, say, regiment-level and consist of platoons of self-propelled anti-aircraft platforms, whether they are SPAAGs, integrated air-defence systems like Tunguska or all-in-one surface-to-air missile platforms like Roland or SA-11 Gadfly.

Air defence by air forces is typically taken care of by fighter jets carrying air-to-air missiles which is beyond the scope of this article, however most air forces choose to augment air base defence with surface-to-air missile systems as they are such valuable targets for enemy aircraft. In addition, countries without dedicated air defence forces often relegate these duties to the air force. For example, the United States' strategic air defence is the domain of the Air Force, even when it is performed by missiles launched from fixed installations. For example, see Project Nike.

Most modern air defence systems are fairly mobile. Even the larger systems tend to be mounted on trailers and are designed to be fairly quickly broken down or set up. In the past, this was not always the case. Early missile systems were cumbersome and required much infrastructure - many could not be moved at all. With the diversification of air defence there has been much more emphasis on mobility. Most modern systems are usually either self-propelled (i.e. guns or missiles are mounted on a truck or tracked chassis) or easily towed. Even systems which consist of many components (transporter/erector/launchers, radars, command posts etc.) benefit from being mounted on a fleet of vehicles. In general, a fixed system can be identified, attacked and destroyed whereas a mobile system can show up in places where it's not expected. Soviet systems especially concentrate on mobility, after the lessons learnt in the Vietnam proxy-war between the USA and USSR. For more information on this part of the conflict, see SA-2 Guideline.

North Korea (officially the DPRK) has inherited a lot of older Soviet equipment. One major reason for the success of the U.N. forces during the Korean War (1950-1953) against the DPRK and PRC was the air superiority they were able to attain. As tensions still exist on the Korean peninsula and the DPRK is so heavily militarised, their air-defence network is amongst the strongest of a non-superpower. A large part of it consists of a number of older, fixed systems like SA-2, SA-3, SA-4 and SA-5. But the DPRK is also in possession of many mobile systems which have proven to be deadly in the past.