After losing her husband, Greenhow became more sympathetic to the Confederate cause. Greenhow was an advocate for secession and "preserving the Southern way of life," including slavery. After passing control of the espionage network to Greenhow, Jordan left the US Army, went South, and was commissioned as a captain in the Confederate Army. On July 9 and July 16 of 1861, Greenhow passed secret messages to Confederate General
P. G. T. Beauregard containing critical information regarding Union military movements for what would be the
First Battle of Bull Run, including the plans of General
Irvin McDowell. Assisting in her conspiracy were pro-Confederate members of Congress, Union officers, courier
Betty Duvall, and her dentist,
Aaron Van Camp, as well as his son who was also a Confederate soldier,
Eugene B. Van Camp. She became known as "Rebel Rose" for her work for the South.
Capture and prison , Washington, D.C., 1862 Knowing she was suspected of spying for the Confederacy, Greenhow feared for her remaining daughters' safety. Leila was sent to Ohio to join her older sister Florence Greenhow Moore, whose husband Seymour Treadwell Moore had become a captain in the Union Army. (He was breveted a brigadier general in May 1865 for his services and achieved a rank of lieutenant colonel after the war in his army career.) Only Little Rose stayed with Greenhow in Washington.
Allan Pinkerton was made head of the recently formed
Secret Service and one of his first orders was to watch Greenhow, because of her wide circle of contacts on both sides of the sectional split. Due to the activities of visitors, he arrested Greenhow and placed her under
house arrest at her 16th Street residence on August 23, 1861, along with one of her couriers,
Lily Mackall. The materials included numerous love letters supposedly from the
abolitionist Republican U.S. Senator
Henry Wilson from Massachusetts. She considered him her prize source, and claimed he gave her data on the "number of heavy guns and other artillery in the Washington defenses," but he likely knew far more from his work on the Military Affairs Committee. The seized Greenhow papers are now held by the
National Archives and Records Administration, with some available online. Contemporary biographers of Wilson have regarded the letters as likely not sent by Wilson as his handwriting and signature do not match those in the letters. Pinkerton supervised visitors to Greenhow's house and moved other suspected Southern sympathizers into it, giving rise to the nickname Fort Greenhow. He was pleased to oversee the visitors and messages, as it gave him more control of the Southern flow of information. While messages continued to be sent to Jordan, he discounted them after Pinkerton mounted his control. When a letter from Greenhow to Seward complained of her treatment was publicized, there was Northern criticism for what was perceived as too lenient treatment of a spy. Pinkerton transferred Greenhow on January 18, 1862, to
Old Capitol Prison, shutting down Fort Greenhow. So many political prisoners were detained that a two-man commission was set up to review their cases at what were called espionage hearings. Greenhow was never subjected to trial. Greenhow also on one occasion flew the
Confederate Flag from her prison window. ==International acclaim==