As Aliyev and Sargsyan flew home, the Nagorno-Karabakh conflict escalated dramatically. In a matter of hours, rules of engagement tacitly developed since a 1994 cease-fire went out the window. For the first time, a mass incursion of tank formations occurred. For the first time, Azerbaijani forces sought to seize and hold territories held by NKR forces. For the first time, GRAD missiles were used and a series of other weapon systems, like armed drones, were thrown into the fight. NKR forces shot down at least one Mi-24 helicopter and destroyed numerous tanks. And, tragically, the death toll in this one eruption is the largest since 1994, with scores dead, possibly more.

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Here are five things you need to know about this longstanding conflict.

1. After World War I, Nagorno-Karabakh was caught between — and claimed by — two emerging nations, Armenia and Azerbaijan.

The conflict over Nagorno-Karabakh started amid the great conflicts of the 20th century. The Ottoman and Russian Empires that had dominated the Caucasus collapsed at the end of World War I. Nationalist parties pitted neighbors — Christian Armenians, Turkic Muslims and others — against each other and tried to define national homelands. Caught in the middle was Karabakh, a multicultural mosaic spread across a largely mountainous terrain.

The Soviet Union, the new power in the region, devised a solution. It created a Nagorno-Karabakh “autonomous oblast” (NKAO) — an oblast is an administrative district — as an island within the Soviet republic of Azerbaijan. It was not contiguous with Armenia. The NKAO’s majority Armenian population wasn’t happy with this. The oblast remained a bone of contention between the Soviet republics of Armenia and Azerbaijan.

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When Gorbachev loosened Soviet power in the mid-1980s, local Armenian nationalist groups fought to change the facts on the ground, polarizing the region. As the Soviet Union disintegrated, Karabakh’s fate became the salient issue in both Armenia and Azerbaijan.

In September 1991, local Armenians proclaimed the Nagorno-Karabakh Republic, defining its territory to include the NKAO and other parts of Azerbaijan, including the Shaumian region to the north and territories to the east of Martakert and Martuni — to be governed by local Armenians, independent of Azerbaijan.

Azerbaijani forces sought to destroy the NKR. Armenian forces, locals and volunteers from Armenia proper sought to secure its existence. The war was ugly. An estimated 750,000 Azerbaijanis were driven from their homes in the fighting, the vast majority not from the NKAO but from surrounding provinces seized by the Armenian forces and from Armenia proper. More than 300,000 ethnic Armenians inside Azerbaijan were forcefully displaced as well.

Initially, the local Armenian NKR treated those seized territories as assets to be traded in a final peace deal. Over time these areas were reimagined as “liberated territories,” part of an organic homeland called Artsakh. The current constitution of the NKR does not define the state borders. But its local maps include the historic NKAO (whose borders are not marked) and the seized territories; other outside territories are marked as “under Azerbaijani occupation.” In return, Azerbaijan contends that all of NKR’s territory is actually Azerbaijan but is under Armenian occupation.

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2. Nagorno-Karabakh is not a frozen conflict. It’s a simmering one.

The Karabakh conflict is commonly described as “frozen.” Observers often categorize it with other contested post-Soviet territories like Transnistria (Moldova), Abkhazia and South Ossetia (Georgia), and, more recently, Crimea and the Donbas (Ukraine).

But this is too simplistic, for three reasons.

First, the war over Karabakh came to an end in 1994 with a cease-fire but without a peace agreement. A “line of contact” separates the parties in the contested Karabakh area; Armenia and Azerbaijan also share a long border to the north of the NKR. There are no international peacekeepers in the area and only a handful of observers from the Organization for Security and Cooperation in Europe (OSCE). The line of contact features World War I-style trenches on both sides, in some places three rows deep.

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Second, there has always been shooting across the line of contact. In recent years, that has included particularly nasty long-range sniper fire and incursions by small groups of soldiers. And the conflict has been heating up: Last year, more soldiers died than in any year since 1994 — and this last weekend’s fighting alone appears to have killed more than died in 2015.

Finally, no Russian troops are stationed in Karabakh, unlike in the other post-Soviet conflicts. Russia remains the most powerful power broker in the Caucasus, guaranteeing Armenia’s security with a military base in Gyumri, in the west of Armenia facing Turkey, while supplying weapons to both sides.

3. Karabakhi residents overwhelmingly oppose compromise

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For a National Science Foundation project on de facto states, we conducted large representative surveys in 2010-12 in four post-Soviet regions.

Karabakh is now ethnically homogeneous — and its respondents were the most uncompromising of all those we surveyed. They were all but united in opposing the return of displaced Azerbaijanis or any shifts in territory and in disliking and distrusting their former enemies. The NKR survey results showed that most people strongly distrusted others and were unwilling to forgive past violence.

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Karabakhis also had the highest levels of ethnic pride among the dozens of ethnicities that we have surveyed in post-Soviet states and the Balkans. In fact, their pride was all but unanimous, with 73 percent of respondents saying they are “very proud” of their identity and another 21 percent describing themselves as (merely!) “proud.”

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And what’s especially important for any discussion of a “land for peace” negotiation with Azerbaijan is this: Karabakhis strongly (85 percent) rejected any notion of a return to the borders of the NKAO of Soviet times. They slightly supported, with just over 60 percent, their current expansive territorial limits, which you can see in the blue shaded areas on the map. But nearly 70 percent preferred a vision of their homeland that adds areas, hatched on the map, still under Azerbaijani control as well as an undetermined and large “historic” area across the south Caucasus.

4. Karabakhis aren’t confident that peace negotiations will succeed — and believe they must be ready to fight for themselves

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Peace negotiations, with Armenia representing the NKR, have stopped and started over the past two decades, making little real progress. Only half of our Karabakhi respondents believed that the discussions, hosted by the OSCE’s Minsk Group (co-chaired by ambassadors from France, Russia and the United States), will succeed. A bare majority of 53 percent thinks that international peacekeepers can help resolve the conflict.

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Rather, Karabakhis have a strong sense that they must look out for themselves and mobilize against Azerbaijani threats. Majorities were worried about an Azerbaijani military buildup (63 percent) and about a new war with their neighbor (58 percent). Still, they believed that they would be able to withstand any attack. Just over a quarter (26 percent) of Karabakhis were willing to even consider ceding land for peace.

5. The Karabakh conflict seems local, but it could drag in major world powers

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The Karabakh conflict seems intensely local, fighting over a few villages and kilometers. But it’s influenced by broader global shifts and regional regime calculations.

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First, declining oil revenues and recession in Russia have placed Azerbaijan’s Aliyev regime under pressure. In December 2015, the government was forced toward a floating exchange rate for its currency, the manat; in one day, the manat plunged 32 percent against the U.S. dollar. Living standards are dropping and street protests are rising. The regime may believe a distracting war spectacle in Karabakh has benefits. Armenia’s democracy-challenged ruling clique may also believe a conflict would help its citizens “rally around the flag.” That’s a doubly dangerous situation.

Second, the chill in relations between Russia and Turkey has opened up splits between Azerbaijan’s pro-Russian and pro-Turkish factions. Armenia is a member of Moscow’s Eurasian Customs Union; Azerbaijan is not, yet. Russia may want to place peacekeepers in the area and to underscore its role as the indispensable power in this region, and its “near abroad” more generally.

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Third, Aliyev’s invitation to D.C., his first visit in a decade, was a symbolic victory in the face of widespread criticism about media suppression and human rights abuses. Did face time with Kerry, Biden and Obama “free” Aliyev to pursue war? We will not know for some time.

So what’s ahead?

The Karabakh conflict has historically unseated governments in both Armenia and Azerbaijan. A cease-fire has now been agreed to after four days of fighting. Will this eruption of violence galvanize international diplomacy, which until now has been small and hasn’t put enough resources on the ground?

Nagorno-Karabakh is a conflict with the potential to escalate quickly into something broader, entangling Russia and Turkey (a member of NATO), and galvanizing the Armenian diaspora. We’ve been warned.

Gerard Toal (Gearóid Ó Tuathail) is the author of “Near Abroad: Putin, the West and the Contest for Ukraine and the Caucasus” (forthcoming, Oxford University Press) and professor of government and international affairs at Virginia Tech’s National Capital Region campus in Old Town Alexandria.