1st Army
Active	July 1914 – beginning of 1918
Country	Russian Empire
Branch	Imperial Russian Army
Type	Field army
Engagements	World War I Eastern Front Battle of Stallupönen Battle of Gumbinnen Battle of Tannenberg First Battle of the Masurian Lakes Battle of Łódź Bug–Narew Offensive Second battle of Przasnysz Narew Offensive
Commanders
Notable commanders	GeneralAleksei Brusilov GeneralLavr Kornilov GeneralPaul von Rennenkampf

The1st Army (Russian:1-я армия,romanized: 1-ya armiya) was anarmy-level command of theRussian Imperial Army created duringWorld War I. The First Army, commanded by GeneralPaul von Rennenkampf,invadedEast Prussia at the outbreak of war in 1914 along with theSecond Army commanded by GeneralAlexander Samsonov.^[1] After declaring war on theGerman Empire, theRussian Empire had been able to mobilize very quickly. All Russian forces were put under the command ofGrand Duke Nikolai and his Quartermaster GeneralYuri Danilov.

The invading forces made a determined and speedy attack on East Prussia. However, the First and Second Armies were stopped by theGerman Eighth Army, led by Field MarshalPaul von Hindenburg and his chief of staff, GeneralErich Ludendorff. The German and Russian armies met atTannenberg, where the Second Army was encircled and suffered complete destruction. Both the First and Second Armies suffered terrible casualties in one of the most comprehensive German victories ofWorld War I. The First Army also suffered defeat at theFirst Battle of the Masurian Lakes in September 1914, which led to Rennenkampf's dismissal and replacement by Litvinov. First Army served under Northwestern front for the remainder of the war. Litvinov was replaced by Sokovnin in April 1917. Vannovski replaced Sokovnin in July and the army's final commander, von Notbek, took over in September 1917.

The First Army consisted of the following units in August 1914:

• Northwestern Front (July 1914 – August 1915)
• Western Front (August 1915 – April 1916)
• Northern Front (April 1916 – July 1917)
• Southwestern Front (July – September 1917)
• Northern Front (September 1917 – the beginning of 1918)

• Battle of Stallupönen (17 August 1914)
• Battle of Gumbinnen (20 August 1914)
• Battle of Tannenberg (23–30 August 1914)
• First Battle of the Masurian Lakes (8–11 September 1914)
• Battle of Łódź (11 November – 6 December 1914)

The 1st Army had the following commanders until it was demobilized in 1918.

• List of Imperial Russian Army formations and units

• Military units and formations established in 1914
• 1914 establishments in the Russian Empire
• Military units and formations disestablished in 1918
• Field armies of Russia in World War I

• Articles with short description
• Short description matches Wikidata
• Use dmy dates from August 2020
• Short description is different from Wikidata
• Articles containing Russian-language text