The Second Chechen War, in a later phase better known as the Counter-terrorist operation on Chechnya, was launched by the Russian Federation starting 26 August 1999, in response to the Invasion of Dagestan by the Islamic International Peacekeeping Brigade (IIPB).

On 1 October Russian troops entered Chechnya. The campaign ended the de facto independence of Chechen Republic of Ichkeria and restored Russian federal control over the territory. Although it is regarded by many as an internal conflict within the Russian Federation, the war attracted a large number of foreign fighters.

During the initial campaign, Russian military and pro-Russian Chechen paramilitary forces faced Chechen separatists in open combat, and seized the Chechen capital Grozny after a winter siege that lasted from late 1999 to the following February 2000. Russia established direct rule of Chechnya in May 2000 and after the full-scale offensive, Chechen militant resistance throughout the North Caucasus region continued to inflict heavy Russian casualties and challenge Russian political control over Chechnya for several more years. Some Chechen separatists also carried out attacks against civilians in Russia. These attacks, as well as widespread human rights violations by Russian and separatist forces, drew international condemnation.

Chechen War may refer to: