During a Thursday appearance on MSNBC’s Morning Joe, Libertarian presidential candidate Gary Johnson was asked about the city of Aleppo, the site of a major front in the Syrian civil war.

Johnson responded with a question: “What is Aleppo?”

This wasn’t some kind of weird dodge — it’s clear he actually doesn’t know what Aleppo is. In a follow-up interview, Johnson was given a chance to explain himself. It didn’t go well:

When you recognize what’s going on in Syria, when you recognize that Aleppo is in kinda the epicenter between ... Aleppo! Umm, knowing that there’s a city between the two forces, really at the epicenter, but not remembering or recognizing that that’s Aleppo ... guilty.

This is deeply embarrassing for the former New Mexico governor, who’s been trying to position himself as the thinking conservative’s alternative to Donald Trump.

But not everyone is running for president like Johnson is, so not everyone is expected to have snap answers handy about the complex Syrian civil war like Johnson should. Which means a lot of you out there probably have the same legitimate question: What is Aleppo, anyway, and why are people asking Johnson about it?

So here’s a brief guide — to both the city’s role in the Syrian conflict and why it’s so important.

What is Aleppo?

Aleppo is a city located in northwest Syria near the Turkish border. It’s the capital of the eponymous Aleppo province, and was the country’s most populated city before the war. Today it’s perhaps the most significant battlefield in the defining conflict in Syria: fighting between Bashar al-Assad and various different anti-government rebel factions. (ISIS is not a major player in Aleppo City, though it has a presence in Aleppo province.)

Since Syria’s 2011 Arab Spring protests descended into a civil war, Aleppo has been a major base for a number of different rebel factions opposed to the Assad regime. In September 2015, Assad's forces began a concerted effort to retake the city. The above map shows the government's progress: By December, they had made significant advances around the city and, by February 5, had nearly surrounded it.

Prior to the Aleppo offensive, Assad had been losing. In response, his international patrons — Russia and Iran — began increasing their support last fall. Their deployments allowed Assad's forces to make their major push toward the city.

"The operations in Aleppo Province have hinged upon heavy military support from both Russian warplanes and Iranian proxy fighters," Christopher Kozak, a research analyst at the Institute for the Study of War, wrote at the time. He continued:

Russia concentrated a significant portion of its air campaign against opposition forward positions and supply lines in Aleppo Province. Meanwhile, U.S. officials estimated in October 2015 that up to 2,000 Hezbollah, Afghan, and Iraqi Shi’a militia fighters led by Islamic Revolutionary Guard Corps (IRGC) – Quds Force commander Maj. Gen. Qassem Suleimani currently operated in Aleppo Province.

The worst-case scenario, at the time, was an all-out siege of the city’s rebel-controlled eastern region, blocking even humanitarian assistance. This sort of siege is illegal under international law, but Assad has been doing it for years. That's because it works: Rebels who don't have food quickly lose the will to fight, and civilians in besieged areas will start cooperating with the government just to make it stop.

In July, these fears were realized. The Assad regime, with significant support from Iran, imposed a blockade on rebel areas, cutting off supplies to some 320,000 people.

At the end of July, the fractious rebel groups struck back, launching a rare coordinated offensive that caught the Assad regime off guard. By August 7, they had broken the siege of the city, allowing aid to flow in.

But the fighting didn’t stop. You may have seen one of the images from the subsequent fighting: a little boy, named Omran Daqneesh, covered in dust and blood. Omran had been caught in a Russian airstrike in the city and was pulled out from under a building by rescuers.

While the rebels aren’t nearly as responsible for Aleppo’s pain, they’re hardly saints themselves.

“The rebels include both jihadists formerly allied with Al Qaeda and commanders aligned with the Free Syrian Army, which has been supported sporadically by the Obama Administration,” the New Yorker’s Steve Coll writes. “The rebels fire inaccurately into government areas with improvised mortars that they call ‘hell cannons.’ The rounds include gas cylinders packed with explosives and metal shrapnel, designed to terrorize and maim.”

More recently, the Assad regime has begun making gains again. On September 4, parts of eastern Aleppo were “re-besieged” by Assad’s forces, taking advantage of divisions among the rebel groups. According to Charles Lister, a Syria expert at the Middle East Institute, Russian special forces were “centrally involved” in directing the offensive.

The Assad regime has advanced further since then, retaking the Ramousah district that rebels had used to establish a supply corridor. Wednesday afternoon, hours before Johnson’s comments, it reportedly unleashed a major chlorine gas attack on a civilian-populated area. At least 120 people suffered gas inhalation; a 13-year-old girl named Hajer Kyali died.

So Aleppo isn’t just one site of tragedy among many in Syria. It’s a humanitarian crisis and one of the most important battlefronts in the country — the kind of thing that someone who wants to be president of the United States should at least have passing familiarity with.

What does the battle for Aleppo tell us about Syria?

This back and forth in Aleppo, where the rebels and Assad seem to trade gains and momentum, illustrates the fundamental seesaw dynamic of the civil war. The city is important not just in humanitarian terms but also because a candidate’s approach to Aleppo is a microcosm for his approach to the entire conflict.

Assad had lost ground nationwide for most of 2015 before the Russian and Iranian escalations helped him launch the Aleppo offensive. For most of the two years prior to that, he appeared to have had the upper hand on the rebels — who, in turn, had looked likely to win for much of 2011 and 2012.

These shifts in momentum reflect the fundamental weakness of all parties.

Assad has manpower problems, the rebels are deeply divided, the Kurds have no ability to control a mostly Arab country, and ISIS has managed to make enemies out of virtually every powerful actor in the Middle East. No side is strong enough to crush any other by dint of force, so gains end up being pretty temporary.

Moreover, both Assad and the non-ISIS rebels are backed by actors outside of the country, who tend to escalate when it looks like their proxies are losing ground. The role of Russian special forces in the recent besiegement of Aleppo, seemingly a response to the rebel breaking of the siege, is a perfect example.

This creates a deadly seesaw effect, whereby Assad and the rebels keep trading territory without anyone ever gaining a permanent upper hand. This also makes any kind of peace deal even harder to negotiate: At any given point, either the regime or the rebels feel like they're winning on the battlefield. Whoever has the upper hand has no incentive to come to terms.

These conditions, and other, make Syria a nearly impossible conflict to resolve even by the grim standard set by other civil wars. “This is a really, really tough case,” Barbara Walter, a UC San Diego expert on civil wars, told the New York Times’s Max Fisher.

Aleppo poses the basic dilemma created by the situation — how should the US handle a war that it can’t end, short of a massive invasion? — in miniature.

Should the United States try to break the Aleppo siege using its own military, and thereby risk open conflict with nuclear-armed Russia? Should it provide deadly weapons to the opposition, knowing that those weapons might make their way to civilian-killing jihadists or simply prolong the conflict? Or should it stay out, and risk hundreds of thousands of people being gassed and starved?

These are the cold, hard realities of Syria policy — perhaps the most difficult foreign policy challenge that our next president will face.

The fact that Johnson doesn’t seem to know the first thing about it is ... well, it would be a bit troubling if he had any chance of winning.