NEW YORK (NYTIMES) - President Donald Trump's decision this week to withdraw all US troops from Syria within 30 days risks leaving the United States' allies in the long-running war weakened while strengthening rivals backed by Iran and Russia.

US troops entered Syria in 2015 as part of a coalition fighting the Islamic State in Iraq and Syria (ISIS), which had seized large swathes of territory in Syria and Iraq. In the three years since, the extremist group's self-declared caliphate has crumbled. But the ongoing lack of stability in both Syria and Iraq could provide fertile ground for the militants to return.

The American pullout could also weaken the country's influence over any negotiations on a settlement to end the conflict.

"The leverage that might have been there for the United States in Syria is no longer there because now everyone knows that the United States will leave Syria unconditionally," said Mr Joost Hiltermann, the Middle East director of the International Crisis Group, a conflict and foreign policy research organisation.

Here are some of the parties to the conflict that have the most to gain or lose from a US withdrawal.

THE WINNERS

Iran, Russia and Syrian President Bashar al-Assad and his chief international backers Russia and Iran would all benefit from a US troop withdrawal, which would further tighten Mr Assad's once-tenuous grip on his battered country.

Iran

Iran is one of the biggest winners as the international ally with the most invested in Syria and the most at stake. During the war, Iran embedded itself in Syria, redrawing the strategic map of the Middle East.

It has sent in thousands of Shi'ite forces, who fought on the ground, and deployed drones and precision weapons to keep Mr Assad in power. That secured an all-important land bridge through Syria to supply weapons to Hezbollah, Iran's Shi'ite militia ally in Lebanon and a steadfast enemy of Israel.

Iran trained and equipped Shi'ite fighters while strengthening ties with allies in Iraq and Lebanon in hopes of building a united front in the event of a new war with Israel.

Russia

Russia also stands to benefit. A day after Mr Trump's announcement on Wednesday (Dec 19), President Vladimir Putin of Russia applauded the decision, saying during a news conference, "Donald's right, and I agree with him."

Russia contributed around 5,000 troops and a few dozen aircraft to prop up Mr Assad's government, which secured Moscow's strategically important naval facility in the Syrian city of Tartus on the Mediterranean Sea. Russia also expanded its military footprint in Syria during the war.

"It certainly helps the Russians, who have benefitted tremendously from a quite limited investment in Syria," said Mr Jon B. Alterman, director and senior fellow of the Middle East Programme at the Centre for Strategic and International Studies.

Through its alliance with Syria, Russia has maintained its influence in the Middle East.

"They re-established themselves as a global player when the conclusion had been that the glory days of the Soviet Union were dead and gone," Mr Alterman said.

Syrian President Bashar al-Assad

For Mr Assad, the US withdrawal means the path forward for Syria will be shaped largely by forces sympathetic to his government and its interests.

The two biggest threats to his leadership have been substantially neutralised - the myriad rebel groups that tried to overthrow the Syrian government and ISIS - the latter thanks largely to the military force brought to bear by the US-led international coalition that fought the militants.

Turkey

Turkey and the US, Nato allies, have frequently found themselves at odds in Syria, even though both opposed Mr Assad. That is because the US backed a mostly Kurdish force in Syria, saying they were the fighters most capable of pushing back ISIS.

Turkey has long battled Kurdish separatists at home in the country's south-east and saw the rising power of Kurds along its border in northern Syria as a threat. President Recep Tayyip Erdogan of Turkey recently threatened military intervention against the Kurdish forces in Syria that Washington has backed since 2015.

The exit of US troops would leave Turkey open to taking action to curb the power of Kurdish forces in Syria.

ISIS

"We have won against ISIS," Mr Trump declared in a video that was published on Wednesday. But experts, including some of Mr Trump's own staff and coalition partners, disagree.

Though the militants retain just 1 per cent of the territory they held at the height of power, this would remove a major military adversary in the region. During a State Department briefing on Dec 11, Mr Brett McGurk, Mr Trump's special envoy in the fight against ISIS, said the battle was not over.

"The end of ISIS will be a much more long-term initiative," Mr McGurk said. "Nobody is declaring a mission accomplished."

THE LOSERS

Syrian Kurds

Despite being key US allies in the fight against ISIS, the Kurdish-led Syrian Democratic Forces are being virtually abandoned, critics of the withdrawal say. The Kurds have relied on US support, and a sudden withdrawal could be disastrous, leaving them exposed from all sides.

The Syrian Democratic Forces denounced the withdrawal in a statement on Thursday.

"The White House's decision to withdraw from northern and eastern Syria will negatively affect the campaign against terrorism," the group said. "The fight against terrorism is not over yet, and the final defeat of terrorism has not come yet."

The group warned that the move would create a "political-military vacuum" that would allow ISIS to thrive again.

Kurdish forces are likely to lose territory and control as a result of Mr Trump's decision.

"Kurds and their allies have paid a very heavy price," said Mr Mutlu Civiroglu, a Washington-based Kurdish affairs analyst. "They have fought on the front line, and thousands of Kurdish men and women lost their lives fighting on behalf of the entire world." He said many now feel betrayed: "They feel like all the efforts are about to go in vain."

As the Kurds - a stateless and often marginalised group - took back territory from ISIS forces in northern Syria, they worked to create an autonomous region.

Israel

A newly empowered Iran with unfettered land access to their Hezbollah allies - without US forces in the north of Syria as a counterweight - poses an existential threat to Israel.

"Israel will be very unhappy about this because they see it as a net gain for Iran, and they are right," Mr Hiltermann said.

As Israel's most powerful ally, the US plays an outsize role in security for the country, and the withdrawal of troops could threaten that balance.

Civilians

Civilians have borne the brunt of the conflict in Syria for years, with millions displaced from their homes and millions more who fled the country struggling abroad as refugees.

Aid groups warn that further destabilisation of northern Syria could spark yet another humanitarian disaster in the region.

The International Rescue Committee, which has been working to provide humanitarian assistance in parts of Syria for years, warned that a potential Turkish offensive in the region could be devastating.

"Throughout this conflict, these political and military decisions have been made without any apparent consideration of the humanitarian consequences. As a result, every decision has heightened the danger and distress for civilians," said Mr David Miliband, president of the International Rescue Committee.

Many Kurdish civilians would likely flee the area if the Kurdish militias lose control of northern Syria.

"There will be a humanitarian crisis, there is no question," Mr Hiltermann said.