On December 19, Donald Trump made a move that took almost everybody, including members of his own administration, off guard – he ordered a full, rapid withdrawal of over 2,000 US troops from Syria.

The president justified his decision by saying that the only reason US troops were in Syria was to defeat the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) group, and now that this mission is accomplished, there is no reason for them to stay in the country.

Trump’s unexpected announcement, which underlined the continued absence of a clear and coherent US strategy in Syria and the wider Middle East, is likely to mark the start of a new period of conflict in the region.

Following Obama’s footsteps

Over the past few years, apart from defeating ISIL, the US has not been able to define clear political objectives in Syria.

Barack Obama was elected on a promise to reverse his predecessor George W Bush’s heavy military involvement in the Islamic world. He hence ordered the full withdrawal of US troops from Iraq at the end of 2011.

In June 2014, Mosul’s fall to ISIL forced Obama to get involved in Iraq once again. A US-led international coalition to defeat ISIL and prevent it from establishing a state across Syria and Iraq was formed. However, Obama was still reluctant to commit a large number of ground forces to this fight, so he relied on local proxies to fight ISIL.

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In Iraq, Obama worked with the Iraqi government, Kurdish Peshmerga and Shia militias against ISIL. In Syria, the Syria Democratic Forces (SDF), the backbone of which is the Kurdish YPG, became America’s most reliable local ally against the armed group. The Obama administration trained, funded and equipped the Kurdish group despite strong objections from Turkey, which considers the YPG a terror organisation.

A full-blown proxy war

Mainly as a result of the Obama administration’s reluctance to act as a hegemon, Syria’s conflict rapidly transformed into a full-blown proxy war. The unwillingness of the US to play a more active role in the conflict enabled regional powers – such as Turkey, Iran and Saudi Arabia – to step in and try to influence the course of events in Syria and the Levant at large. Russia also joined the fray in September 2015, when it became absolutely clear that the US had become utterly uninterested in the outcome of the Syrian conflict.

When Trump moved into the White House in early 2017, despite his known disapproval of most of his predecessor’s policies, and the well-known reluctance of some members of his administration to end the US military presence in the Middle East, he chose to continue with Obama’s hands-off approach in the region.

And only a year later, he expressed his intention to go even further than Obama and order a full withdrawal of US troops from Syria.

Trump first announced that the US will be “coming out of Syria, like very soon” in March 2018. Regional allies and advisers convinced the US president that ISIL was not completely defeated, so he agreed to give the Pentagon and State Department another six months to finish the job, still refusing to commit to an open-ended military presence in Syria.

Implications for Syria and the region

Now that the Trump administration officially announced its intention to leave Syria for good, regional powers who have been active participants in Syria’s war will likely increase their efforts to gain control of the areas that are currently under US control.

As things stand now, the US, through its Kurdish allies, controls approximately one-third of Syrian territory. These areas are justifiably dubbed by the media and analysts as “useful Syria”: They contain Syria’s major oil and gas fields, main water resources, dams, power plants and most of its fertile land.

Regaining control over these territories is of vital importance for Russia. Moscow lacks the funds to sustain major reconstruction efforts in post-conflict Syria, without which its costly military achievements – defeating the opposition and securing the regime of President Bashar al-Assad – would be hollow. It also wants to be financially rewarded for its military support of the Syrian regime. Hence it had long been eyeing the oil and gas fields that are currently under US control. Now that the US is leaving, Russia will do everything necessary to be the power that fills this vacuum.

Iran is also interested in the US-controlled Syrian territories, albeit for completely different reasons. Since the US invasion of Iraq in 2003, Iran has been working hard to establish a “Shia Crescent” from western Afghanistan to the Mediterranean Sea. The US withdrawal from Iraq in 2011 brought Iran one step closer towards achieving that goal. Yet, the rise of ISIL and the loss of a huge swath of territories in eastern Syria and western Iraq to the group denied Iran the possibility of keeping a land corridor open from Tehran to Damascus and to Hezbollah in Lebanon. Iran supported the US war on ISIL and even sought membership in the international coalition against the armed group, with the expectation that the US will leave the region once this fight is over. Now that the US is doing just that, Iran will resume its efforts to have the trans-Syria land corridor reopened by trying to increase its influence over northeast Syria.

The US decision to leave northeastern Syria will also cause problems for Israel. In September, a Russian spy plane was downed by Syrian regime forces after Israeli jets used it for cover during attacks in Syria. The incident caused Moscow to downgrade its cooperation with Israel in Syria. As a result, Tel Aviv became fully dependent on the US to keep Iran’s influence in Syria in check. Following the US decision to leave Syria, Israel is now left with little leverage to shape events on the ground in Syria.

Saudi Arabia also has strategic interests in the area. Over the past year, Riyadh exerted tremendous efforts to convince President Trump to maintain a substantial military presence in northeast Syria to counterbalance both Turkey and Iran. Last November, the Saudis committed $100m to convince the US to keep its troops in Syria. At one point, Riyadh even offered to send troops to patrol the area alongside the US and the YPG. Hence the US decision to leave the area likely caused major disappointment for the Saudis and encouraged them to play an even more hands-on role in the country’s future.

Turkey too is interested in this part of Syria. It has long accused the US-backed SDF of trying to establish an independent state in northeast Syria and has called repeatedly for the US to end its support for the Kurdish group, which it considers to be the Syrian arm of the PKK. In recent weeks, Turkey threatened to launch a major crossborder military operation to destroy its bases in Syria. Now that the US is withdrawing its troops from the region, it might be tempted to move in and eliminate the YPG as it did with Operation Olive Branch in Afrin early in the year.

This means the SDF is possibly the actor that will be most affected by the US decision to withdraw from Syria. Now that it is officially abandoned by its superpower patron, the Kurdish group will be forced to start looking for new allies to help it survive in the new political environment. Most likely, it will move closer to the Russia-Iran-Syrian regime axis to deter a Turkish military intervention.

ISIL might also find a window for a resurgence in the vacuum that will be created as result of Washington’s exit.

In light of all this, the US withdrawal from Syria is likely to be the single most important development in the Syrian conflict since Russia’s intervention in September 2015. It could bring the Syrian conflict back to where it was before the rise of ISIL: a major power play fueled by the competing interests of regional actors. We might hence witness another round of conflict in Syria between middle size powers after the departure of the hegemon. In other words, the ultimate outcome of Trump’s decision to leave Syria could perhaps be the start of a new “all-against-all” war in the Middle East.

The views expressed in this article are the author’s own and do not necessarily reflect Al Jazeera’s editorial stance.