Plunder or pillage always incident to war, and, whatever rules exist for restraint, the conflict usually leads to authorized devastation and plunder, retaliatory to exhaust the enemy. For instances, in Civil War of 1861-65, Sherman's destruction of property in march through Southern territory, Sheridan's destroying agents in the Shenandoah Valley.
By Hague rule of 1899, July 29, pillage of a town or place even when taken by assault is prohibited.
How about Allies in Pekin?
See Instructions to United States Army in the field. General Orders, April 24, 1863, War of Rebellion:—
All wanton violence committed against persons in the invaded country, all destruction of property not commanded by the authorized officer, all robbing, all pillage and sacking even after taking a place by main force, all rape, wounding, maiming or killing of such inhabitants are prohibited, under penalty of death or such other severe punishment as may seem adequate to the gravity of the offence.
A soldier, officer, or private may be killed by superior officer for such act. See John Bassett Moore's "Digest of International Law."
Brown was more outspoken than General Wayne. See "Major-general Anthony Wayne, and the Pennsylvania Line," by Charles J. Stillé, President Historical Society of Pennsylvania. J. B. Lippincott Company, 1893. (Pages 235 et seq.)
General Anthony Wayne to H. A. Sheel.