Warre attended Royal Military College, Sandhurst in the summer of 1807 and the following May was appointed "aide-de-camp" to Major-General Sir Ronald Craufurd Ferguson. A few months later, Warre returned to his native Portugal where he saw action at the battles of Roliça (17 August) and Vimeiro (21 August) before falling ill with dysentery. He was sent to recover in Lisbon, where Major-General William Carr Beresford welcomed him into his house. After his recovery, Beresford installed Warre in his staff and he served throughout Sir John Moore's campaign, ending with the Battle of Coruña in January 1809.