Fierce firefights and heavy shelling echo once again around the mountains of Nagorno-Karabakh, an isolated region at the very edge of Europe that has seen several major wars since the fall of the Soviet Union.

On Tuesday, the South Caucasus nation of Azerbaijan announced its armed forces launched “local anti-terrorist activities” in Nagorno-Karabakh, which is inside Azerbaijan’s borders but is controlled as a breakaway state by its ethnic Armenian population.

Now, with fighting raging and allegations of an impending “genocide” reaching fever pitch, all eyes are on the decades-old conflict that threatens to draw in some of the world’s leading military powers.

What is happening?

For weeks, Armenia and international observers have warned that Azerbaijan was massing its armed forces along the heavily fortified line of contact in Nagorno-Karabakh, preparing to stage an offensive against local ethnic Armenian troops. Clips shared online showed Azerbaijani vehicles daubed with an upside-down ‘A’-symbol, reminiscent of the ‘Z’ sign painted onto Russian vehicles ahead of the invasion of Ukraine last year.

In the early hours of Tuesday, Karabakh Armenian officials reported a major offensive by Azerbaijan was underway, with air raid sirens sounding in Stepankert, the de facto capital. The region’s estimated 100,000 residents have been told by Azerbaijan to “evacuate” via “humanitarian corridors” leading to Armenia. However, Azerbaijani forces control all of the entry and exit points and many locals fear they will not be allowed to pass safely.

Azerbaijani President Ilham Aliyev’s top foreign policy advisor, Hikmet Hajiyev, insisted to POLITICO the “goal is to neutralize military infrastructure” and denied civilians were being targeted. However, unverified photographs posted online appear to show damaged apartment buildings, and the Karabakh Armenian human rights ombudsman, Gegham Stepanyan, reported several children have been injured in the attacks.

Concern is growing over the fate of the civilians effectively trapped in the crossfire, as well as the risk of yet another full-blown war in the former Soviet Union.

How did we get here?

During the Soviet era, Nagorno-Karabakh was an autonomous region inside the Azerbaijani Soviet Socialist Republic, home to both ethnic Armenians and Azerbaijanis, but the absence of internal borders made its status largely unimportant. That all changed when Moscow lost control of its peripheral republics, and Nagorno-Karabakh was formally left inside Azerbaijan’s internationally recognized territory.

Amid the collapse of the USSR from 1988 to 1994, Armenian and Azerbaijani forces fought a grueling series of battles over the region, with the Armenians taking control of swathes of land and forcing the mass exodus of hundreds of thousands of ethnic Azerbaijanis, razing several cities to the ground. Since then, citing a 1991 referendum — boycotted by Azerbaijanis — the Karabakh-Armenians have unilaterally declared independence and maintained a de facto independent state.

For nearly three decades that situation remained stable, with the two sides locked in a stalemate that was maintained by a line of bunkers, landmines and anti-tank defenses, frequently given as an example of one of the world’s few “frozen conflicts.”

However, that all changed in 2020, when Azerbaijan launched a 44-day war to regain territory, conquering hundreds of square kilometers around all sides of Nagorno-Karabakh. That left the ethnic Armenian exclave connected to Armenia proper by a single road, the Lachin Corridor — supposedly under the protection of Russian peacekeepers as part of a Moscow-brokered ceasefire agreement.

What is the blockade?

With Russia’s ability to maintain the status quo rapidly dwindling in the face of its increasingly catastrophic war in Ukraine, Azerbaijan has moved to take control of all access to the region. In December, as part of a dispute supposedly over illegal gold mining, self-declared “eco-activists” — operating with the support of the country’s authoritarian government — staged a sit-in on the road, stopping civilian traffic and forcing the local population to rely on Russian peacekeepers and the Red Cross for supplies.

That situation has worsened in the past two months, with an Azerbaijani checkpoint newly erected on the Lachin Corridor refusing to allow the passage of any humanitarian aid, save for the occasional one-off delivery. In August, amid warnings of empty shelves, malnourishment and a worsening humanitarian crisis, Luis Moreno Ocampo, the former chief prosecutor of the International Criminal Court, published a report calling the situation “an ongoing genocide.”

Azerbaijan denies it is blockading Nagorno-Karabakh, with Hajiyev telling POLITICO the country was prepared to reopen the Lachin Corridor if the Karabakh-Armenians accepted transport routes from inside Azerbaijani-held territory. Aliyev has repeatedly called on Armenian forces in Nagorno-Karabakh to stand down, local politicians to resign and those living there to accept being ruled as part of Azerbaijan.

Why have things escalated now?

Over the past few months, the U.S., EU and Russia have urged Azerbaijan to keep faith during diplomatic talks designed to end the conflict once and for all, rather than seeking a military solution to assert control over the entire region.

As part of the talks in Washington, Brussels and Moscow, Armenian Prime Minister Nikol Pashinyan made a series of unprecedented concessions, going as far as recognizing Nagorno-Karabakh as Azerbaijani territory. However, his government maintains it cannot sign a peace deal that does not include internationally guaranteed rights and securities for the Karabakh-Armenians.

Aliyev has rejected any such arrangement outright, insisting there should be no foreign presence on Azerbaijan’s sovereign territory. He insists that as citizens of Azerbaijan, those living there will have the same rights as any other citizen — but has continued fierce anti-Armenian rhetoric including describing the separatists as “dogs,” while the government issued a postage stamp following the 2020 war featuring a worker in a hazmat suit “decontaminating” Nagorno-Karabakh.

Unwilling to accept the compromise, Azerbaijan has accused Armenia of stalling the peace process. According to former Azerbaijani Foreign Minister Elmar Mammadyarov, a military escalation is needed to force an agreement. “It can be a short-term clash, or it can be a war,” he added.

Facing growing domestic pressure amid dwindling supplies, former Karabakh-Armenian President Arayik Harutyunyan stood down and called elections, lambasted as a provocation by Azerbaijan and condemned by the EU, Ukraine and others.

Azerbaijan also alleged Armenian saboteurs were behind landmine blasts it says killed six military personnel in the region, while presenting no evidence to support the claim.

What’s Russia doing?

Armenia is formally an ally of Russia, and a member of the Collective Security Treaty Organization (CSTO) military bloc. However, Russian peacekeepers deployed to Nagorno-Karabakh have proven entirely unwilling or unable to keep Azerbaijani advances in check, while Moscow declined to offer Pashinyan the support he demanded after strategic high ground inside Armenia’s borders were captured in an Azerbaijani offensive last September.

Belarusian dictator Alexander Lukashenko previously said Azerbaijan has better relations with the CSTO than Armenia, despite not being a member, and described Aliyev as “our guy.”

Since then, Armenia — the most democratic country in the region — has sought to distance itself from the Kremlin, inviting in an EU civilian observer mission to the border. That strategy has picked up pace in recent days, with Pashinyan telling POLITICO in an interview that the country can no longer rely on Russia for its security. Instead, the South Caucasus nation has dispatched humanitarian aid to Ukraine and Pashinyan’s wife visited Kyiv to show her support, while hosting U.S. troops for exercises.

Moscow, which has a close economic and political relationship with Azerbaijan, reacted furiously, summoning the Armenian ambassador.

In a message posted on Telegram on Tuesday, Dmitry Medvedev, former president of Russia and secretary of its security council, said Pashinyan “decided to blame Russia for his botched defeat. He gave up part of his country’s territory. He decided to flirt with NATO, and his wife took biscuits to our enemies. Guess what fate awaits him…”

Who supports whom?

The South Caucasus is a tangled web of shifting alliances.

Russia aside, Armenia has built close relations with neighboring Iran, which has vowed to protect it, as well as India and France. French President Emmanuel Macron has previously joined negotiations in support of Pashinyan and the country is home to a large and historic Armenian diaspora.

Azerbaijan, meanwhile, operates on a “one nation, two states” basis with Turkey, with which it has deep cultural, linguistic and historical ties. It also receives large shipments of weaponry and military hardware from Israel, while providing the Middle Eastern nation with gas.

The EU has turned to Azerbaijan to help replace Russia as a provider of energy. European Commission President Ursula von der Leyen made an official visit to the capital, Baku, last summer in a bid to secure increased exports of natural gas, describing the country as a “reliable, trustworthy partner.”