Forced out of the Chalice following the Chalice Cluster Uprising, Andre Warnecke and his fugitive followers occupied the planet and the Marsh System. He mounted a local reign of terror and organized widespread piracy operations throughout the Silesian Confederacy. When Captain Honor Harrington of the Royal Manticoran Navy learned of these operations and the location of Warnecke's base at Marsh in early 1910 PD, she moved against him and his followers.
In the subsequent First Battle of Marsh, she and the crew of HMAMC Wayfarer destroyed Warnecke's forces, captured Warnecke himself, and prevented his use of nuclear demolition charges on the cities of Sidemore. Warnecke's operations ended and a small detachment established to deal with the stragglers of his forces, Harrington chose to surrender Warnecke to the reconstituted Sidemorian authorities for trail and execution.
Harrington further recommended that the Star Kingdom of Manticore establish a fleet base at Sidemore, since it was close to but not part of the Silesian Confederacy, and on the side nearest the Andermani Empire, with whom tensions had been rising. It was a treaty relationship that would eventually lead to the creation of the Sidemore Navy. (HH6, HH10)
At the beginning of the Second Havenite-Manticoran War in 1920 PD, Marsh and Sidemore was the site of one of the opening engagements of Operation Thunderbolt between the Second Fleet of the Republic of Haven Navy led by Admiral Lester Tourville and Task Force 34 of the RMN led by Admiral Honor Harrington, supported by Task Force Sidemore and the Protector's Own Squadron of the Grayson Space Navy. (HH10)