 Complete the following phrase:  On 17 February 1781, Gustav was born the fourth son of Frederick V, Landgrave of Hesse-Homburg and his wife, Landgravine Caroline of Hesse-Darmstadt. Gustav joined the Swedish army in service of his godfather King Gustav III, but left soon after in favour of an Austrian regiment. The prince saw action in the early 19th-century battles of Nördlingen, 