He graduated the Helsingfors Junker Infantry School [ru] in Helsinki and began his military career with the Lithuanian 5th Lancers Regiment.

In late September 1886, he was the chief of staff of the Warsaw Military District, serving under General and later Field Marshal Count Gurko.

Rennenkampff, with a small detachment of three arms, defeated the Chinese in a strong position along the ridge of the Small Hingan and, having overtaken his infantry, with fourteen hundred Cossacks and a battery, having made 400 kilometers in three weeks with continuous skirmishes, captured a large Manchurian city of Qiqihar with a sudden raid.

But, without waiting for the detachment to be collected, General von Rennenkampff, taking with him 10 hundred Cossacks and a battery, on August 24 moved forward along the Sungari valley; On the 29th he captured Boduna, where 1,500 boxers surrendered unawares to him without a fight; September 8, seized Kaun-Zheng-tzu, leaving here five hundred and a battery to ensure its rear, with the remaining 5-hundred, having done 130 km per day, flew to Jilin.

A handful of Rennenkampff Cossacks, lost among the mass of the Chinese, for several days, until reinforcements arrived, was in a preoriginal position ... – Anton Denikin[9]In mid-September, Rennenkampf left for Dagushan, leaving hundreds of troops to protect the mint and arsenal.

In the Battle of Mukden, Rennenkampf again distinguished himself commanding the Tsinghechensky detachment, which was stationed at the left flank of the 1st Manchurian Army led by General Nikolai Linevich.

In his memoirs, which refer again to rumors, Hoffmann mentioned that both generals quarrelled in Liaoyang after the battle; however, this claimed argument remains physically impossible, as Rennenkampf was seriously injured at this time.

After the war, Rennenkampf commanded an infantry battalion with several machine guns which followed a train in Harbin, restoring communications between the Manchurian Army and Western Siberia.

Within this time, the Revolution broke out; this task remained interrupted via a revolutionary movement in Eastern Siberia, the Chita Republic.

In late October 1906, he faced an assassination attempt while walking along the street with two of his adjutants; a member of the Socialist Revolutionary Party, who was sitting on a bench, threw a bomb onto the general's feet.

While retreating upon orders from the defending 8th Army commander, General Maximilian von Prittwitz, a gap formed between the Russian armies; exploiting this, the Imperial German 1st Division, commanded by General Hermann von François, counterattacked at the Battle of Stallupönen, initiating the Eastern Front of the war.

When he finally began to head south, he and his men started too late and proceeded more slowly than they should have,[citation needed] contributing to the defeat of the 2nd Army.

On 7 September, the First Battle of the Masurian Lakes began, as the Germans attacked the left flank of the 1st Army with a powerful detachment.

Zhilinsky broke his promise to provide Rennenkampf with reinforcements from other formations; as a result, the 1st Army had to retreat hurriedly.

In mid-November at Łódź, due to the indecisiveness and mistakes made by Ruzsky, the 1st Army was unable to prevent the XXV Reserve Corps of General Reinhard von Scheffer-Boyadel from escaping out of the encircling movement, causing the front to retreat.

His acts during the battle became the subject of a special commission under General Peter von Baranoff; he was even considered being tried for treason, due to his ethnic background.

However, his retirement was extremely unsettling, as he was falsely accused of being a traitor, and all across Russia, on streets and in public places, people insulted him for his actions.

are traitors and that the Germans are being given a move, as well as the mood of military censorship letters convinced that the appointment of Pavel Plehve, with some of his regime and perseverance to carry out operations even with victims, prompted (Grand Duke Nikolai) to abandon the original idea of Plehve and to dwell on a man with a Russian surname.In a further investigation, it was revealed that it was Ruzsky's strategic mistakes that had let Scheffer-Boyadel and his troops escape from the encirclement.

Rennenkampf was arrested shortly after the February Revolution, as many of the revolutionaries remembered his role in the suppression of the Chita Republic back in 1905.

As a result, Rennenkampf was taken hostage by the Bolsheviks and brought near the railway tracks running from the Martsevo station to the Baltic.

In May 1918, after the Whites had taken over Taganrog, a stick was found stuck in the ground near the railway tracks running from the Martsevo station to the Baltic Plant.

In recent years, researchers at the Hoover Institution archives of the Stanford University found several photos taken during the civil war.

This allowed historians and researchers to determine and establish the final resting place of Rennenkampf at the burial ground in the Old Cemetery in Taganrog.

In Vera's memoirs, she wrote of her founding of the Committee for Assistance to the families of reserve lower ranks, tailoring workshops for the front, and her participation in the formation of the flying car detachment of the Evangelical Red Cross that took the wounded from the battlefield.

Vera was not alone in considering her husband to be innocent of the strategic mistakes that led to the Russian defeats in the East Prussian campaign; a significant part of the White émigrés, including generals Baron Wrangel, Anton Denikin, and Nikolai Golovin, shared that view.

The honorary committee included the widow of Baron Wrangel – Baroness Olga Mikhailovich, General Nikolai Epanchin, Prince Belosselsky-Belozersky and others.