The von der Deckens are an old noble family in Lower Saxony which produced several officers for the forces of the Electorate of Hanover. In 1784 Johann Friedrich joined the army, fighting in the French Revolutionary Wars from 1793 to 1795. He headed a military journal with his friend Gerhard von Scharnhorst, the Prussian military reformer for many years. In 1803 he joined the diplomatic service on Napoleon's occupation of Hanover and left for the United Kingdom. On 28 July 1803 he began together with Sir Colin Halkett recruiting expatriate Hanoverian soldiers to fight for the United Kingdom in the War of the Third Coalition and subsequent Napoleonic conflicts, with his recruits forming the King's German Legion, with which he fought from 1805 to 1807.