September 15, 2006 Yet another Leonardo da Vinci invention conceived hundreds of years before its time, the armoured tank began its impact on world military history 90 years ago today when it was first deployed by British troops to break the stalemate of trench warfare during the Battle of the Somme. Named because of its resemblance to a water tank, the first prototype (bottom left) was developed in England in secrecy under the sponsorship of Winston Churchill and the first 49 tanks powered by 105 hp Daimler engines were shipped from England to France after the crews had been trained in secret, and the first tank attack in history commenced on the morning of September 15, 1916. Though 31 broke down almost immediately, six got bogged (top right pictured with German troops), eight were hit by German artillery and two caught fire, two of the 3 mph (yes, three) metal monsters achieved major breakthroughs and tank warfare was born, forever changing ground warfare.
The tank precipitated the initially cumbersome anti-tank mine, which was so easy to dig up and redeploy against its makers that the smaller, harder-to-detect anti-personnel mine was born, becoming a modern scourge. Ironically, the German Army against which the armoured tank was first deployed, used the tank as one of the cornerstones of its highly effective blitzkreig ("a war as fast as a lightning"), a tactic which emanated from a thinktank of senior German officers who evaluated the lessons from the country's loss of WW1. The current main battle tank of the U.S. Army is the Abrams (bottom right), and a worthwhile read if you're into tanks can be found in this article, entitled "Heavy Metal - A Tank Company's Battle to Baghdad"
There's an excellent article on the subject on the BBC site, and as usual, Wikipedia has the granular detail on this fascinating subject.