From 1808 until 1813 Halkett fought in the Peninsular War, except in 1809 when he took part in the Walcheren Expedition. He fought at the Battle of Albuera in Charles Alten's independent KGL brigade. When his brother was promoted to lead the brigade, Halkett took over command of the 2nd Light Infantry Battalion, KGL. At the Battle of Salamanca, his battalion fought in John Hope's 7th Division. In the Siege of Burgos campaign, he distinguished himself at the Battle of Venta del Pozo. In 1813 he joined the new Hanoverian army. At the Battle of Göhrde he led a brigade of Hanoverian troops in Count Wallmoden's army. He captured a Danish standard at the action of Sehestedt.