Between 1915 and 1917 he served as Tsar Nicholas II's Chief of Staff of the Stavka, and after the February Revolution, was its commander-in-chief under the Russian Provisional Government from March to May 1917.

He later played a principal role in founding the Volunteer Army in the Russian Civil War and died in 1918 of heart failure while fighting the Bolsheviks in the Volga region.

[2]: 20 In 1890 with the rank of lieutenant colonel, he was posted as a senior adjutant in the headquarters of the 1st Army Corps in the St. Petersburg Military District.

The beginning of World War I saw Alekseyev appointed Chief of Staff to N. I. Ivanov, Commander of the Southwestern Front[2]: 20  (which held the Third, Fourth, Fifth, and Eighth Armies), where he planned the Russian offensive into Galicia with the rank of General-of-Infantry.

He was, however, incapable of changing a political system that allowed the promotion of incompetent commanders through nepotism, patronage and the use of court intrigue.

However, the Russian Army did not manage to exploit strategic benefits of the successful offensive and the situation at the front did not improve significantly.

[4] On 13 November, after the October Revolution, Alekseyev fled Petrograd, arriving Novocherkassk on 15 Nov. With the support of the Don Cossacks Ataman, General Alexey Kaledin, he formed the Alekseev Organization, which would become the core of the anti-Bolshevik Volunteer Army.

On 31 December, Kornilov took military command of the Volunteer Army, while Alekseyev took responsibility for civil authority, foreign affairs and finances.

After the death of Kornilov, Alexeyev appointed Denikin commander of the Volunteer Army, successfully completing the Ice March.