Rennenkampf, of Baltic German extraction, was born in 1854 in Konuvere Manor now in Märjamaa Parish, Rapla County, Estonia. He joined the Imperial Russian Army at 19 and attended the Junker Infantry School in Helsinki. He began his military career with the Lithuanian 5th Lancers Regiment. He graduated at the head of his class from the Nikolaevsky Military Academy in St. Petersburg in 1882. He was assigned as a junior officer to the 14th Army Corps until 1884, and to the Warsaw Military District followed by the Kazan Military District to 1886. Rennenkampf was subsequently a senior aide to the headquarters of the Don Cossacks from March 1888, staff officer to the 2nd Army Corps from October 1889, and Chief of Staff of the Osowiec Fortress from March 1890. The same year he was promoted to colonel. From November 1899, Rennenkampf, as chief of staff of the Transbaikalia region, was promoted to major general.
Rennenkampf commanded four infantry battalions, two Cossack sotnias, and two horse batteries, during the Boxer Rebellion in 1900-01, and was responsible for the capture of Tsitsihar and Kirin in Manchuria, thereby removing any Boxer threat to Harbin and the Chinese Eastern Railway. During this campaign, he was decorated with both the 4th degree and 3rd degree Order of St. George for military distinction.
After the rebellion, Rennenkampf was offered a command in the Imperial Guard, but to the Tsar's surprise, he turned it down. He was then given command of the 1st Independent Cavalry Brigade, a post he held until 1904.
In February 1904, after the outbreak of the Russo-Japanese War, Rennenkampf was appointed commander of the Trans-Baikal Cossack Division. He was promoted to lieutenant general in July 1904, and was wounded in combat in the same month, during the Battle of Motien Pass (10 July 1904), where he was shot in the thigh as he attempted to assist Lieutenant General Fedor Keller halt the advance of Japanese First Army. He remained hospitalized until after the Battle of Liaoyang (August/September 1904). He returned to active service, commanding a division-size cavalry formation during the Battle of Shaho (October 1904), and a corps-sized force during the Battle of Mukden (February/March 1905). At Mukden he replaced Major General Mikhail Alekseyev in command of the Russian left flank after a series of defeats and managed to stabilize the situation. However, after the Battle of Mukden, General Alexander Samsonov accused Rennenkampf of failing to assist him during the fighting, and the two quarreled.
After the Russo-Japanese War, Rennenkampf restored part of his reputation dealing with revolutionaries in Siberia as commander of the Seventh Siberian Army Corps. His suppression of the Chita Republic and re-taking of the Trans-Baikal Railroad, which had fallen into the hands of mutinous soldiers led to his promotion to full general in December 1910; however, in following years revolutionaries targeted him for assassination several times. From 1912, he was promoted to Adjutant General and appointed Chief-of-Staff of the Vilno Military District.
World War I
At the outbreak of World War I, Rennenkampf was placed in command of the Third Army, which took part in the opening stages of the war against Austria Hungary. In August 1914 he was one of the officers leading the Russian forces during the offensive in Galicia.