On Wednesday, images surfaced of militants from the Islamic State of Iraq and Syria (ISIS) driving U.S.-made humvees across the Iraqi border, into the Syrian province of Deir Ezzor. If real, the photos prove that the border between Iraq and Syria is now an open road for ISIS fighters hoping to establish a Sunni caliphate in the region. To the leaders of the extremist group, the battles in Iraq and Syria are part of a single, broader fight.

Analysts say the financial and strategic spoils of ISIS’s capture of Mosul and Tikrit could provide a significant, nearly unstoppable boon to its Syrian arm, helping turn the tide in the months-long battle for Deir Ezzor.

“The weapons and money that they’re gaining through the takeover of Mosul and other areas in Iraq can be used not only to consolidate what they’re doing in Iraq, but to send money back into Syria, both to their operation in Deir Ezzor and to push further into the northern part of country, Aleppo and Idlib, where they’d been operating but been pushed back,” says Aaron Zelin of the Washington Institute for Near East Policy, who studies the Syrian jihad.

Since late March, Deir Ezzor has seen relentless fighting between ISIS and their main rivals for supremacy in eastern Syria, the al-Qaida-linked Jabhat al-Nusra. Control of the sprawling oil fields in the resource-rich province is a top priority for both groups; in April, the Carnegie Endowment estimated that Syrian oil sales, much of them from the province, are still hitting up to $50 million per month. Some of the oil from Deir Ezzor is smuggled to markets in Aleppo, implying that it transits through ISIS-controlled territory in Raqqa.

Since its emergence in Syria in 2012, ISIS has been moving fighters, weapons and goods across the Iraq-Syria border. Between Hassakeh province, on the Syrian side, and Nineveh in Iraq, it has effectively dominated land routes since last summer.

North of Deir Ezzor, “the border has been porous for some time, and ISIS has been able to use it with impunity,” says Aymenn al-Tamimi, a fellow at the Middle East Center who studies Syrian military dynamics. “If they capture the province, it makes it all the easier for them to move freely between Iraq and Syria.”

The synergies of a growing presence in Syria and a consolidating base in Iraq have strengthened the group’s hand, in both countries.

“The spoils from Iraq definitely give them additional military and financial resources to devote here. The tide of battle has turned to ISIS as they push deeper into Deir Ezzor.”

“I’m not of the camp who says the border is gone, but it’s definitely not well enforced at the moment,” says Valerie Szybala, an analyst at the Institute for the Study of War who focuses on Deir Ezzor and the eastern provinces.

“The Syrian regime has little to no presence there, and they’re not going to step up and do anything about this now. Iraqi security forces are not up to the task. The only bulwark against ISIS now is the Kurds [from neighboring Hassakeh],” she said.

“ISIS really has a lot of freedom of movement right now. You can’t [normally] just roll a tank from Iraq into Deir Ezzor through an official border checkpoint.”

For ISIS, the fusing of controlled territory from Deir Ezzor to Mosul is a major step towards its stated goal of creating a unified Sunni caliphate in the region.

“The conflicts in Iraq and Syria have long been fusing,” says Peter Harling, the Damascus-based project director for the International Crisis Group’s Middle East program. “ISIS operates across the border and steps up its activities on one side when it feels either empowered or under pressure on the other. The frontier line is eroding.”

The organization, now considered more dangerous than al-Qaida by many Western officials, is run by Abu Bakr al-Baghdadi, a merchant’s son regarded as the ideological heir to late al-Qaida leader Osama bin Laden.

In Syria, ISIS has made the city of Raqqa its de facto capital, taking over the city last year. But it has struggled in recent campaigns to expand the area it controls.

ISIS faltered its bid to wrest control of terrain from Jabhat al-Nusra and other rebel groups, and its attempts to implement an ultra-conservative form of religious law has been met with resistance from Syrian civilians.

Women are now forced to wear head-to-toe Islamic dress when out in public, while cigarettes and Western products like Coca-Cola have largely been banned. Earlier this year, civilians in Raqqa began to protest after the reported hangings of Syrian journalists and activists, accused of criticizing ISIS rule. Across the north and east, the group’s violent tactics both on and off the battlefield have made them increasingly unpopular.

In January, ISIS emerged victorious from clashes against rebel forces led by Nusra and the Islamic Front, keeping control of Raqqa city. But the infighting forced it out of northern provinces and slowed its advance into Deir Ezzor.

“We’ve seen them do well in Syria before, in Idlib and Aleppo, and then there’s backlash and they get kicked out,” Zelin says. “They have a lot of enemies everywhere. So far, because of the general destabilization in Syria and Iraq, they’ve been successful. But that doesn’t mean they’ll have momentum.”

For now, the group is claiming that it’s already reaping the rewards of this week’s Iraq advance.

“On Twitter, they’ve had pictures that they claim are American tanks from Iraq being brought into Deir Ezzor and inspected by their leaders there,” Szybala says. “They claim they are already getting benefits from the spoils of Iraq.”

The gains could push the Syrian army into action against ISIS. Analysts say that Iran, President Bashar al-Assad’s most important regional ally, is insisting that he take action. Iran, a Shiite power, and ISIS, a Sunni insurgency, have rival goals in the power battle of Syria’s war.

Szybala says there are signs the regime is beginning to take on ISIS, attacking the group’s strongholds in Raqqa and Hassakah earlier this week.

“If that’s really happening, I don’t think it’s just for show,” Szybala says. I think it’s probably because this is [partly] Iran’s game, and now that ISIS is proving itself to have real military power and to be a trans-national threat, Iran can’t afford that and would be pushing the regime to finally take action. ISIS is acting like an army now.”