Armenia and Azerbaijan, two ex-Soviet republics in the Caucasus, are locked in a decades-long territorial dispute. Deadly fighting erupted on Sunday, killing as many as 100 soldiers on both sides, with Armenia saying that it had lost 49 soldiers and Azerbaijan saying 50 had died.
It is the worst flare up of violence since September 2021 — when nearly 8,000 people died in a short but bloody war. At the heart of the stand-off between Yerevan and Baku is the contested Nagorno-Karabakh region.
The new fighting has left Moscow — which has good relations with both sides — scrambling to mediate for a truce.
Where is Nagorno-Karabakh?
The mountainous, landlocked area lies in the Karabakh mountains — its name literally means "mountainous Karabakh". Its greatest expanse covered about 8,000 square kilometres. The multi-ethnic former Oblast, or region of the Russian Empire, is about 100 kilometres east of Armenia's border.
Russian Empire collapse
The Soviet authorities merged the predominantly ethnic Armenian territory with Azerbaijan in 1921.
After the 1991 collapse of the Soviet Union, Armenian separatists seized it in a move supported by Yerevan.
An ensuing war killed 30,000 people and forced hundreds of thousands from their homes.
Despite a ceasefire mediated in 1994 by Russia, the US and France, peace negotiations struggle to move forward and fighting erupts frequently.
The latest clashes on Sunday involved Azerbaijan and Armenian separatists accusing each other of igniting the fighting that left both sides with casualties, including civilians.
It followed a flare-up along the border in July, which claimed the lives of 17 soldiers from both sides.
In April 2016, some 110 people were killed in the most serious fighting in years.
Armenia, a Christian country since the 4th century, has been rocked by political and economic instability since it gained independence from the former USSR.
The country's post-Soviet leadership was largely beholden to the interests of Russia and accused of blocking opposition to its rule and tampering with election results.
In the spring of 2018, mass street protests brought current Prime Minister Nikol Pashinyan to power. He has since clamped down on corruption and introduced popular judicial reforms.
Muslim-majority Azerbaijan, on the Caspian Sea, has been under the authoritarian grip of a single family since 1993.
Heydar Aliyev, a former officer of the Soviet security services, the KGB, ruled the country with an iron fist until October 2003. He handed over power to his son, Ilham, weeks before his death.
Like his father, Ilham has quashed all opposition to his rule and in 2017 made his wife, Mehriban, the country's first vice president.
Turkey, with ambitions to be a regional power broker in the Caucasus, has thrown its weight behind oil-rich and Turkic-speaking Azerbaijan.
Their alliance is fuelled by a mutual mistrust of Armenia, and Ankara routinely issues strongly worded statements in support of Baku's ambitions to reclaim Nagorno-Karabakh.
Yerevan harbours hostility towards Turkey over the massacres of some 1.5 million Armenians by Turkey under the Ottoman Empire during the First World War.
More than 30 countries have recognised the killings as genocide, though Ankara fiercely disputes the term.
Russia, which maintains close ties with Armenia, is the major power broker in the region. It leads the Collective Security Treaty Organisation military alliance of ex-Soviet countries that includes Armenia.
Yerevan relies on Russian support and military guarantees because its defence budget is overshadowed by Azerbaijan's spending on arms.
Azerbaijan has recently begun leveraging oil revenue as part of an attempt to overhaul its image in the West.
Baku has invested in massive sponsorship deals including with the Euro 2020 football championship.
Four matches were held in the capital last year, after the tournament was postponed in 2020 due to the coronavirus pandemic.
Azerbaijan has held Formula One grand prix races since 2016.
It has also tried to pitch itself to European countries as an alternative energy supplier to Russia.
On the international stage, Armenia has a vast and influential diaspora that fled during the Ottoman-era repressions.
Reality TV star Kim Kardashian, the late singer Charles Aznavour, and pop star and actress Cher all trace their roots to Armenia.
Some have appointed themselves unofficial ambassadors, like Kardashian who has been outspoken on the issue of the Armenian genocide.