In January 1915, Yudenich was promoted to General of Infantry and replaced Count Illarion Ivanovich Vorontsov-Dashkov as commander of the Caucasus Campaign.

Yudenich was given a free hand by the Grand Duke, and, in September the Russians retook Van and re-established the Administration for Western Armenia in June 1916.

In the summer of that year, his forces fought off a Turkish counterattack, culminating in the Battle of Erzincan, which included the presence of Ottoman General Mustafa Kemal.

In Helsinki, Yudenich joined the "Russian Committee," which had formed in November 1918 to oppose the Bolsheviks, and he was proclaimed as leader of the White movement in northwestern Russia with absolute powers.

In the spring of 1919, Yudenich visited Stockholm, where he met with diplomatic representatives of Britain, France, and the United States and tried with limited success to obtain assistance in developing a Russian volunteer corps to fight the Bolsheviks.

In June 1919, Yudenich made contact with Admiral Aleksandr Kolchak's All-Russian Government based in Omsk, which subsequently acknowledged him as commander-in-chief of all Russian armed forces operating against the Bolsheviks in the Baltic Sea and in northwestern Russia.

The six tanks were supplied by Britain, together with their volunteer crews, who were the only British ground troops to fight alongside the Northwestern Army.

[citation needed] The Northwestern Army generally believed in a united multinational Russia and opposed separatists wanting to create nation-states.

On 19 October 1919 his troops reached the outskirts of Petrograd, but his forces failed to secure the vital Moscow – Saint Petersburg Railway, which allowed the Revolutionary Military Council to send in massive reinforcements to prevent the fall of the city.

Yudenich's stalled offensive collapsed in late October, and the 7th and the 15th Red Armies repulsed the White Russian troops back into Estonia in November.

Politically, the Bolsheviks secured a separate armistice with Estonia on 3 January by promising to recognize Estonian independence, an offer contrary to the White Army's and the Kolchak government's position.

[6] On 28 January 1920, General Stanisław Bułak-Bałachowicz, together with several Russian officers and the Estonian Police, arrested Yudenich as he tried to escape to Western Europe.