Unfortunately, even with all the progress, man still hasn't been able to put an end to warfare. However, technology developed is often dual use – that which is designed during or for a war can have peaceful uses (the internet was originally designed during the Cold War to carry America's nuclear attack codes) and vice versa. Hatfield has played a significant role in defending Great Britain and her interests – during WWI, WWII and the Cold War.
Tank – field tests of the world's first operational tank were carried out in the grounds of Hatfield House in 1916. Among the VIPs who attended demonstrations of this new weapon were King George, Viscount Horatio Kitchener (secretary of state for war but killed a few months later), Arthur Balfour (former prime minister), David Lloyd-George (became prime minister that year), and Winston Churchill (future prime minister). In recognition of the event, after the war, Churchill gifted a Mk I Tank to the then Marquess of Salisbury, who put it on display in Hatfield Park. This rarity is now on display at the Tank Museum in Bovingdon.
DH.98 Mosquito – ranks alongside the Spitfire, Hurricane and Lancaster as one of the most famous British aircraft of WWII. Designed as a private de Havilland project by a team from Hatfield operating in the seclusion of nearby Salisbury Hall, London Colney (now a private residence once more but is bordered by the de Havilland Aircraft Heritage Centre – formerly the Mosquito Aircraft Museum). The team was led by R E Bishop, and had a scare when a German agent was captured in the area (please see Espionage). In total, they built four prototypes at Salisbury Hall. The first prototype W4050 (which is on display at the DHAHC) was trucked to Hatfield for its maiden flight. On 25 November 1940, with Geoffrey de Havilland Jr at the controls and John E Walker next to him, W4050 (at the time designated E0234) took off from Hatfield.
An immediate success, the DH.98 Mosquito proved to be one of the most versatile aircraft of WWII. The 2 October 1940 bombing of de Havilland's Hatfield factory caused some delay to production and led to the dispersal of production. In total, by the time production ended in 1950, 7,781 DH.98 Mosquito aircraft had been built – over 3,000 at the Hatfield factory.
Blue Jay – later called Firestreak. First fully guided launch of this air-to-air missile was made in 1954. Research and development for this guided missile was carried out at Hatfield. Production was carried out at Lostock. It became the RAF's standard AA missile.
Blue Streak – potential dual use missile – as an Inter-Continental Ballistic Missile (ICBM) or as a satellite launcher.
Other missiles developed at Hatfield include Sea Dart, Sea Eagle, Martel (Anglo-French), and ALARM.
Servicing and refurbishment of NATO frontline missiles was also carried out at Hatfield during the Cold War years.