Henry John Temple, 3rd Viscount Palmerston, (20 October 1784 – 18 October 1865) was twice Prime Minister of the United Kingdom in the mid-19th century. Palmerston dominated British foreign policy during the period 1830 to 1865, when Britain stood at the height of its imperial power. He held office almost continuously from 1807 until his death in 1865. He began his parliamentary career as a Tory, defected to the Whigs in 1830, and became the first prime minister from the newly-formed Liberal Party in 1859. He was highly popular with the British public. David Brown argues "an important part of Palmerston's appeal lay in his dynamism and vigour."