The 2016 Nagorno-Karabakh conflict, also known as the Four-Day War, April War, or April clashes, began along the Nagorno-Karabakh line of contact on 1 April 2016 with the Artsakh Defence Army, backed by the Armenian Armed Forces on one side and the Azerbaijani Armed Forces on the other.

The clashes occurred in a region that is disputed between the self-proclaimed Republic of Artsakh and the Republic of Azerbaijan. The region includes the former Soviet Nagorno-Karabakh Autonomous Oblast and surrounding areas, which are integral part of the Republic of Artsakh according to its Constitution. Azerbaijan claimed to prevent purported continuous Armenian shelling of civilian areas in Azerbaijan and started a military operation for this purpose. However, there was no evidence of Armenian shelling. Until the 2020 Nagorno-Karabakh war, the clashes were the worst since the 1994 ceasefire agreement signed by Artsakh, Azerbaijan and Armenia.The scale of the military actions, the number of forces and combat equipment involved, such as heavy artillery, including the use of cluster munitions, tanks, air forces and suicide drones, as well as the statements of Azerbaijani officials clearly indicate that the events of 2–5 April were not a spontaneous escalation, but a carefully planned and prepared military operation, aimed at resolving the Karabakh conflict by the use of force instead of peaceful means.A ceasefire was reached on 5 April between Azerbaijan and Armenia in Moscow. The Nagorno-Karabakh authorities also welcomed the oral agreement. After the agreement, both sides accused each other of violations. Azerbaijan claimed to have regained 20 km2 (7.7 sq mi) of land, while Armenian officials suggested a loss of 8 km2 (3.1 sq mi) of land of no strategic importance. However, the International Crisis Group reported that those heights were of strategic importance.Officially Baku reported the loss of 31 servicemen without publishing their names. Nevertheless, Armenian sources claimed much higher numbers varying between 300 and 500. The Ministry of Defence of Armenia reported the names of 92 military and civilian casualties, in total.The US State Department estimated that a total of 350 people, both military and civilian, had died. Official sources of the warring parties put those estimates either much higher or much lower, depending on the source.