The Islamic State’s capture of the town of Palmyra is another irreparable blow to one of the world’s most important cultural sites. According to reports from Syria, Islamic State fighters are already damaging antiquities, spreading concerns that ISIS will demolish a heritage that has survived for 2,000 years.

Over the past two days, more than 400 people have been killed in the town, and thousands more have been arrested by ISIS forces after Syrian army soldiers fled, even though they were better armed.

That’s how the Islamic State, shortly after capturing the city of Ramadi in Iraq, raked in huge amounts of ammunition and weapons stored in Syrian army depots. ISIS captured a logistical stronghold for the Syrian forces.

But as with the takeover of Ramadi, the conquest of Palmyra is part of a regional strategy to connect the Iraqi and Syrian fronts into a single entity. A look at the map explains the strategy: contiguous arteries between Ramadi in the east via the Syria-Iraq border crossing at Tanf. That’s the last crossing held by regimes forces — and from there it’s on to Palmyra, which straddles an important crossroads.

Palmyra sits on the main road to Deir al-Zur and Raqqa, which are under Islamic State control in the north, and the main road between Deir al-Zur and the major city of Homs in the west. Control of these roads cuts the Syrian army off from the main areas it still controls in the west. It leaves it only the western road network linking Damascus to the Qalamoun Mountains, and from there to the Latakia district, considered the regime’s main stronghold.

In the east, the capture of Ramadi blocks the Iraqi army and the Shi’ite militias under Iranian command from assisting the Sunni tribes. They also can’t fight to liberate Anbar Province, which borders Syria.

The method of capturing main roads and crossroads has been the Islamic State’s strategy since the beginning of the conquest of Iraq in June 2014. Back then, following the takeover of Mosul, Islamic State forces quickly took the main roads leading to western Iraq and the border crossings. They created regional contiguity before moving on to take the cities themselves.

This strategy requires the members of the Western coalition, especially the United States, to treat Iraq and Syria as one front rather than two separate theaters, each with its own rules. The United States regards Iraq as a battle zone outside the control of Baghdad, Tehran or Moscow, while the Syrian front is thought to require the coalition to tread lightly to avoid igniting an all-out conflict.

This distinction plays into the Islamic State’s hands, because ISIS takes advantage of restrictions on engaging it in Syria. It has thus created two logistical and economic fronts, one in Syria and one in Iraq. Even if one falls, the other will be able to supply it using the roads under its control.

That’s also why airstrikes on Islamic State installations aren’t too effective. U.S. Senator John McCain has said that 75 percent of the planes taking off to attack ISIS return with their bombs because of a lack of targets. McCain supports ground assaults with the participation of thousands of U.S. soldiers.

But even if a ground war is launched — such a decision seems unlikely at the moment — the Islamic State has already put up a defensive wall by connecting the two fronts, which makes cooperation between the Iraqi and Syrian armies very difficult.

ISIS’ next decision will thus be where to advance. Should it press east from Ramadi to Habbaniya and Baghdad’s outskirts, or extend its control to Syria, take Homs and from there on to the Lebanese border? The Islamic State must maintain itself economically, so it must keep on taking urban areas to profit from tax, commerce and ransom payments. It can’t waste too much time and effort on open areas.

Therefore Homs in Syria is likely to be a greater temptation than Habbaniya in Iraq. Moreover, on the Syrian front, the Islamic State can take advantage of the rivalries among the militias, and between them and the Syrian army and Hezbollah.

This array of forces could also force the Western coalition to change its policy toward the rebel militias. It could legitimize some of them, like the Al-Qaida-linked Nusra Front, to rally against the Islamic State.