From April 5, 1904, during the Russo-Japanese War, Stackelberg was commander of the Russian 1st Siberian Army Corps, which played a major role in the Battle of Te-li-Ssu. Hampered by orders from Russian commander-in-chief General Aleksei Kuropatkin not to commit his entire reserves, and to fight a defensive battle rather than push forward in an offense, his forces were decisively defeated by the Second Japanese Army under General Oku Yasukata. In the subsequent Battle of Sandepu, Stackelberg chose to ignore Kuropatkin’s orders and made gains against entrenched Japanese positions, albeit with heavy casualties, but was forced to withdraw when reinforcements were denied. Relieved of his command for insubordination, he was sent back to St Petersburg after the battle, where he was awarded the Order of St. George (4th class) for his actions.