In the space of a few weeks in January–February 2018, Israel claimed to have destroyed half of Syria's air defences in response to the bringing down of an Israeli F-16 fighter aircraft; the United States conducted airstrikes, killing Russian 'mercenaries' near Deir al-Zour; and Turkey threatened the US over its military support for the Syrian Kurds.

Iranian expansionism and likely miscalculation of Israeli commitment to 'red lines' pose a high risk of war resulting in major damage to Israeli, Lebanese, and Syrian infrastructure. The Iranian launch of an unmanned aerial vehicle (UAV) over Israel on 9 February, and the unprecedented scale of Syria's response to Israel's retaliatory airstrikes, which resulted in the bringing down of an Israeli F-16 jet, indicate an increased risk appetite on the part of the Iranian/Hizbullah axis. The next war would be likely to involve not only Hizbullah and Israel, but also Syrian government forces, which would result in significant damage to Israeli infrastructure and residential areas, as well as crippling damage to Lebanese and Syrian infrastructure, including bridges, power stations, ports and airports. We assess that Israel's military strategy is still to delay a war as long as possible, while maintaining the Israeli air force's freedom of movement to collect intelligence over both Lebanon and Syria.

Direct conflict between Russia and the United States in Syria remains very unlikely. The US has demonstrated its resolve to hold the River Euphrates de-confliction line, and its commitment to support the Syrian Democratic Forces (SDF), of which the Syrian Kurdish People's Protection Units (Yekîneyên Parastina Gel: YPG) form a major part. The likely aim of the US is to use territory captured east of the Euphrates to contain Iran and extract political concessions from President Bashar al-Assad, including some form of autonomy for the Syrian Kurds, but the level of ground forces currently deployed is insufficient to achieve these objectives by combat, if the US military presence does not deter further Syrian government offensives. The Syrian government, backed by Russia and Iran, has repeatedly declared its intent to re-establish control over all its sovereign territory, including its main oil-producing region. Further Syrian government attempts to take SDF-held territory east of the Euphrates are likely to risk US, as well as further Russian and Iranian/Iranian proxy, casualties, particularly from airstrikes. A deliberate and sustained conflict in northeastern Syria between Russia and the US remains very unlikely, however, given its unacceptably high cost, relative to the secondary importance, in global terms, of both governments' objectives in Syria.

Turkey is unlikely to act on its threat to attack US troops deployed alongside the Kurds in Manbij. Syrian Kurdish YPG forces currently resisting the Turkish incursion into Afrin district in Operation Olive Branch do not have US military support on the ground. However, Turkey has stated its intention to expand its operation east to include the town of Manbij, where US military personnel are present alongside the YPG. The US is likely to strengthen its visible military presence, with the aim of deterring a Turkish offensive, but Turkey's President Recep Tayyip Erdoğan has threatened the US with an "Ottoman slap" if it does not drop its military support for the YPG. We assess that the Turkish armed forces are unlikely to mount an offensive targeting the US presence in Manbij, but that there is a high risk of fighting between Turkish-backed Free Syrian Army (FSA) forces and the YPG, leading to further deterioration in US-Turkish relations.US Secretary of State Rex Tillerson visited Ankara on 15 February and some de-escalation measures were agreed, but it was not clear ifa these would be implemented by or acceptable to the US's Kurdish allies.

Turkey's attempts to deter a Syrian government advance into the opposition-held Idlib province risk drawing Turkey into an escalation with Iran's proxies. Turkey has deployed troops into Idlib province, ostensibly to set up observation posts, as agreed at talks in Astana, Kazakhstan, but we assess primarily to pre-empt an advance by the Syrian government and Iranian proxies, seeking to break the siege on the Shia villages of Kefraya and Foua in Idlib province, and to take Idlib city, the last provincial capital in opposition hands. On 31 January, Turkish troops came under attack by Iranian proxies operating in the area and further fighting between them is likely.

Indicators of changing risk environment

Increased risk

• Redeployment of Iranian proxy forces, including Hizbullah, from Idlib and Deir al-Zour provinces back to Lebanon and southern Syria, indicating increased readiness for war with Israel.
• Iran launches further unmanned aerial vehicles (UAVs) into Israeli airspace, prompting an escalating response by Israel, with further extensive air strikes aimed at neutralising Syria's air defences.
• Russia announces the deployment of Russian government soldiers, as opposed to deniable Russian 'mercenaries', alongside pro-Syrian government forces along the Euphrates River, warning the United States that further airstrikes against its allies will not be tolerated.
• The Syrian government repeats its claim that that the US military presence in eastern Syria is illegal, and that it will, backed by its allies, mount a major offensive on multiple fronts to reclaim its sovereignty, including its oil fields.
• Israel assassinates a senior Hizbullah or Syrian commander, in retaliation for downing its jet fighter.
• The Syrian government again attempts to bring down an Israeli jet using the same tactic that brought down the previous one.
• Turkish and the FSA forces take Afrin and advance on Manbij, despite US warnings.
• Turkey accuses Iran and the Syrian government of providing weapons to the Kurds of Afrin.
• Turkey claims that Syrian-Kurdish YPG militias based east of the Euphrates are responsible for terrorist attacks inside Turkey.
• Kurdish-Arab localised fighting breaks out east of the Euphrates.

Reduced risk

• Iran withdraws Hizbullah, its other proxies, and Islamic Revolution Guards Corps (IRGC) personnel in southern Syria to Damascus.
• A UN Security Council meeting on the Turkish intervention in Syria leads to a Security Council resolution calling for Turkey's suspension of its offensive operations against the YPG, if not withdrawal.
• Russia retains tight control over Syrian airspace over Afrin district, using air-space closures to force Turkey to limit the scope of its operation, probably to the establishment of a 30-kilometre buffer zone along the border.
• The US and Turkey agree on setting limits on Operation Olive Branch, including the extent of Turkish penetration and the duration of the operation.
• The Kurds withdraw from Manbij, and a joint US-Turkish force is deployed there, in accordance with Turkish proposals.