AAA - 'Anti-Aircraft Artillery', also known as 'ack-ack
', furthermore known as 'flak
' and 'aa
' - dates back almost as far as the heavier-than aeroplane; further, if one counts 19th century attempts to shoot down observation balloons, attempts which were hampered by the fact that hot air balloons, when shot, gently deflate rather than bursting.
Aeroplanes, however, do not deflate. Although early biplanes were mostly canvas stretched over a wood framework, a bullet striking a vital area - such as the engine or the pilot - could bring the whole aircraft down, pilot and all. But hitting an aircraft with a rifle was nigh-on impossible; unless the biplane was heading directly towards or away from the rifleman, at a sufficiently low altitude for the bullet to reach, it was a waste of ammunition. Even a lucky hit might go straight through the fabric and out the other side. Machine-guns had a better chance of bringing an aircraft down; but the term 'AAA' derives from a more successful device, that staple of the great war, the artillery piece.
Modified to fire upwards, with the shells fused to burst at a specific height, artillery pieces could fill the sky with lethal shrapnel and high explosive concussion. A direct hit on an enemy aircraft was not necessary; and with several guns filling the sky with spall, pilots could no longer loiter over enemy trenches. They could hardly loiter over friendly trenches either, as the chances of being shot down by one's own side - a constant of aerial warfare from WW2 to the Falklands to the Gulf War - were greatly increased.
AAA was actually developed just before the Great War in Germany, mainly for use against enemy balloons. Like so many technical developments of the time, AAA's potential was not initially recognised, as aeroplanes had not yet proved their worth in combat. Although bombing raids on England by Zeppelins and German bombers - such as the famous Gotha - were ineffective in a military sense, they had enormous psychological value, and by the end of the war Britain and its allies possessed hundreds of anti-aircraft pieces.
In the inter-war period development of AAA stood still, and developments in WW2 (such as the famous German '88', designed as a multi-role anti-aircraft/anti-tank gun in order to circumvent the Treaty of Versailles, which forbade Germany from developing artillery) are covered in more detail in flak. The use of anti-aircraft artillery and cannon against ground troops was frowned upon at the start of that conflict, although never actually barred by the Geneva Convention. Within a few years, it was commonplace, as was rock music, although the two developments are not connected.
From the 1950s onwards AAA was supplemented - although it has never entirely been replaced - with surface-to-air missiles (SAM). Whilst both the European Union and the United States of America have largely ditched AAA, Russia, China, and most of the Middle East and rest of the world still use anti-aircraft guns (of which the Russian ZSU-23 Shilka and the American M163 Vulcan PIVADS, and derivatives of same, are the most visible). They are cheaper and particularly useful against attack helicopters and light strike aircraft. Against a modern, well-funded, opponent, AAA suffers from being relatively static; even mobile AAA pieces have to stay still in order to fire.
A note on terminology: although AAA originally referred only to large-calibre explosive projectiles - anti-aircraft cannon (above 12.7mm calibre; typically 20mm) and machine-guns (12.7mm or less, typically .303 or .308) were just 'aa guns' - constant use and blending of the terms used in the first line of this article mean that 'anti-aircraft artillery' is now a blanket term for all guns (as opposed to rockets) directed at the skies.