This week the US-led coalition struck pro-Syrian regime forces that had attacked a base of the Syrian Democratic Forces. The major battle, which lasted all night into February 7, left more than 100 dead on the Syrian regime side.Using overwhelming firepower, the coalition was sending a message: Stay on your side of the line. On the coalition side of the line, the US is increasingly setting down roots in Syria. Humanitarian aid is starting to flow into the Euphrates River valley, and the battle for the support of local Sunni Arab tribes is taking place against the backdrop of a conflict over who will run post-ISIS Syria.Before Islamic State came to the Euphrates River valley, this was an area that was separated from the high life in Damascus. In the early 2000s it was a pipeline for foreign fighters and terrorists flowing into Iraq from abroad via Syria. The Sunni Arab tribes made money from the smuggling. Some had kin across the border in Iraq and liked Saddam Hussein more than the Assad regime.According to leaked diplomatic cables, during the Anbar awakening and the US-led surge after 2006 against the terrorists, the Americans were able to recruit 25 tribes in Iraq, many of whom had friends across the border. This network of support for the US mostly fell apart by 2014, when ISIS swept down the same Euphrates valley into Iraq.Now, after three years fighting ISIS, the Americans have returned, but this time to eastern Syria rather than Anbar province in Iraq. They understand that the tribes will be a key to security. Some of the US commanders from the coalition saw combat during the surge and are familiar with the way in which they were able to get support in 2006 by working with the tribes.Fabrice Balanche of the Washington Institute for Near East Policy wrote in 2016 that any push to liberate Raqqa from ISIS would need to involve the Arabs of the area. “[The coalition] must rally the tribes in the area, some of whom have pledged allegiance to ISIS. Any such effort will require a thorough understanding of the evolving role that tribes have played there, first under the Assad regime and now under ISIS rule.”Working with the SDF, which comprises both Kurds and Arabs, the US liberated Raqqa in the fall of 2017 and defeated ISIS in 99% of its “caliphate” in Syria.In October Brett McGurk, the US presidential envoy for the war on ISIS, met with sheikhs from the tribes near Raqqa. He wrote that it was an “excellent meeting” and would help to restore life in the area.Akil Hussein, writing on Syria Deeply in January 2018, sketched out the changing loyalties of the tribes, which he estimates make up more than 70% of those in the area. In 2012, when the rebellion began, many of the “clan elders” in the area initially sided with the regime “due to the privileges they had previously received from it.” Then some of them supported ISIS, and later some went to join the SDF. Others stayed with the regime. He pointed to one sheikh from the Baggara clan as an example.Turkey, which supports the Syrian rebels in the north, also wants to play an influential role among Arab tribes in parts of Syria, and Turkey deeply opposes the US working with the SDF, because Turkey accuses the People’s Protection Units, which are part of the SDF, of being “terrorists.”Mona Alami at Al-Monitor writes that “both Russia and the regime on one side and the Kurds on the other are attempting to lure the Deir ez-Zor tribes into their orbit.” Deir ez-Zor is the largest city in the Euphrates valley and was the site of some jockeying for position between the US-backed SDF and the Russian-backed regime in the fall of 2017. The SDF grabbed the Conoco gas fields in September, while the regime got back the city, which had been under siege by ISIS for two years, in November. An uneasy “de-confliction” hangs in the air near the city and up and down the valley, where the river is the dividing line.LIMITED AID has been arriving in eastern Syria over the years, but in July 2017 the US began a more serious effort. By August the US State Department’s START Forward operation was seeking partners to provide aid to 324,000 displaced people, according to an article at Daily Sabah. “McGurk said once Raqqa is cleared of Daesh, START Forward plans to provide food for 447,000 people and shelter up to 50,000.”However, there are complications getting the aid in. Turkey doesn’t want aid going into northeastern Syria from its side, because it believes the aid will end up with the YPG. So aid flows through Iraq, mostly from the Kurdistan region.The US understands that aid is a key to diplomacy in northeast Syria. In December 2017, for instance, House Foreign Affairs Committee chairman Ed Royce has systematically supported sanctions against the Assad regime and legislation to ensure that US aid does not strengthen the regime. This is an important factor because the regime was able to get the World Food Program to air-drop food into Deir ez-Zor to help the city survive the siege from ISIS. That was a worthy goal while fighting ISIS. But in any contest between the US and the regime in eastern Syria, the regime would obviously like to use its existing contacts in international aid organizations and contracts to put its people’s fingers on the spigot.US AID is getting more deeply involved in eastern Syria. “We’re at the point where people really do want to go home, so this is the moment to seize,” Mark Green, administrator of the US Agency for International Development, told Reuters in late January.He made an unannounced visit to Raqqa in Syria with US CENTCOM Gen. Joseph Votel. A map of US AID operations in the eastern Syria provinces of Raqqa, Hasaka and Deir ez-Zor shows layers of US AID programs, including the Office of Food for Peace and the Office for Foreign Disaster Assistance. Support involves humanitarian coordination, risk management, agriculture, economic recovery, shelter and a raft of other assistance.According to the coalition’s Maj.-Gen. James Jarrard, there is a plethora of work to be done in eastern Syria. This involves not only the continuing fight against ISIS and its remaining cells, but also removing IEDs and clearing dangerous ordnance. He speaks about “long-term reconstruction” and “multiyear efforts.”THESE ARE all indications that the US is there for the long haul, something that Secretary of State Rex Tillerson and Secretary of Defense Jim Mattis have indicated. Defending against the pro-regime fighters is one thing, but for the US to set down roots, it will embark on a major effort to rebuild eastern Syria.This is an unprecedented commitment and one of the first of its kind, where the US is operating in a country whose regime does not want it there.The US has engaged in other humanitarian operations and interventions, whether in Haiti, Somalia or Kosovo in the 1990s or Iraq and Afghanistan in the 2000s. But northeast Syria appears to be different. It looks a bit like what the US did in northern Iraq in the 1990s, which helped protect Kurds and eventually led to the creation of the autonomous Kurdistan Regional Government.However, in eastern Syria the US wants to go beyond just a Kurdish canton and invest heavily in the Euphrates River valley. As it does so, the Syrian regime and its Russian and Iranian allies will be there, watching and probing for weakness.