He completed the Nikolaevsky Engineering School in 1883 and, five years later, graduated from the General Staff Academy. Zaionchkovsky saw combat in the Russo-Japanese War in command of a regiment, then a brigade, of infantry.
In 1912 he was promoted to the command of the Thirty-seventh Infantry Division. It was in this role that he entered World War 1. By early 1915 he led a corps.
The Rumanian campaign of 1916 brought Zaionchkovsky into the limelight. Russian military leaders like General Mikhail Alekseev, the chief of staff, had long considered it useless, and perhaps dangerous, to draw Rumania into the war. The Balkan nation's weak army and indefensible strategic situation made it inevitable that it would draw away Russian resources. Alekseev was determined to avoid this, and all the more so in the summer of 1916, when Russia was conducting a promising offensive under General Brusilov north of the Carpathians.
French cajoling brought Rumania into the war in August 1916. Alekseev grudgingly gave his new ally a token force of three divisions. These were to occupy the border province of Dobrudja; it was hoped their presence might deter Bulgaria, Rumania's unfriendly southern neighbor, from declaring war on it. Zaionchkovsky received the thankless task of commanding this so-called Dobrudja Detachment.
The situation seemed to Zaionchkovsky to promise disaster. His force was too small to be effective. The Bulgarians might hesitate to make war on Russia, which many Bulgarians saw as a traditional friend; but Zaionchkovsky's scratch force included a Serbian division, and the Bulgarians and the Serbs were historic antagonists. After voicing these concerns to Alekseev, Zaionchkovsky heard himself called a coward and received orders to get on with his assignment.
Calamity struck soon after Zaionchkovsky reached the Dobrudja. Bulgaria declared war on Rumania in early September. A motley force of German, Bulgarian, Austrian, and Turkish troops under Field Marshal von Mackensen invaded the Dobrudja at once. Zaionchkovsky found his Rumanian allies prone to leave his flanks open at moments of maximum danger. The Rumanian authorities, in turn, saw their Russian comrades in arms all too willing to ravage the countryside in their spare moments. Despite Mackensen's numerical disadvantage, Zaionchkovsky found himself pushed out of the important port of Constanza by late October. Meanwhile, the German Ninth Army under General von Falkenhayn was sweeping into Rumania from the west. As the Russo-Rumanian alliance buckled under the strain of repeated military failures, Zaionchkovsky was relieved. All the same, Alekseev found himself drawn deeper into the quagmire. By year's end, thirty-six Russian divisions had been dispatched to help the Rumanians hold at least the corner of their country bordering on Russia.
Zaionchkovsky survived the debacle. On his return to Russia, he received command of the XVIII Corps. In 1917 he was promoted general of infantry; then, in the wake of the March Revolution, he was pensioned off. A final chapter in Zaionchkovsky's military career began in 1918, however, when he joined the Red Army. He served as chief of staff for the Thirteenth Army against the White forces led by General Denikin. At the conclusion of the Civil War, he immersed himself in military history. He had written widely on the subject even before 1914; he was then given the task of leading the Red Army commission studying the lessons of World War I. He himself wrote prolifically on the subject, while teaching at the Red Army Military Academy.
His works on the war in which he had played such an uncomfortable role in 1916 are still considered valuable contributions to historical scholarship. Zaionchkovsky died in Moscow, March 22, 1926.
While a professor, "Zaionchkovski worked as an agent for the Soviet secret police at the same time he was head of the conspiratorial anti-Bolshevik Monarchist Union (The Trust)."