Here is a look at some of the likely effects of President Donald Trump's decision to pull out the 2,000 US troops as he declared the defeat of the Islamic State group:

- Kurdish fighters and Turkey -

Among the most alarmed at a US pullout will be Kurdish fighters who form the backbone of the Syrian Democratic Forces, an opposition force that has seized about a quarter of the country with Washington's backing.

Turkish President Recep Tayyip Erdogan vowed this week to "remove" the Kurdish People's Protection Units, which he sees as linked to the Kurdish Workers' Party, or PKK, the force that has waged an insurgency inside Turkey since 1984.

Turkey had earlier been forced to tread cautiously in Syria, with any injury to US troops certain to trigger a crisis.

Hours after Trump's announcement, the United States said it had approved a $3.5 billion missile package for Turkey, a NATO ally that had earlier angered the United States by signing an arms deal with Russia.

- Can ISIS return? -

Kurdish fighters who were on the frontlines of fighting the Islamic State movement, often known as ISIS, will certainly shift focus if they come under attack from Turkey.

The United States has not announced an end to its air war in Syria, but it would be relying on significantly less intelligence without troops on the ground.

Critics of Trump's decision noted that ISIS sprouted in Iraq after former president Barack Obama, also eager to end a foreign intervention started under his predecessor, withdrew.

Ilan Goldenberg, a former US diplomat now at the Center for a New American Security, said that a successor to ISIS could similarly re-emerge, prompting a fresh US intervention.

"We're about to make the exact same mistake in the Middle East that we've been making again and again for the past 20 years," he wrote on Twitter.

- Reinforces Russia and Iran -

As the United States withdraws, Assad's allies Russia and Iran have shown no sign of leaving.

Russia sees longtime ally Syria as a strategic asset in its quest to restore a global role, while Iran's Shiite clerical state sees a religious imperative in fighting Sunni hardliners and protecting President Bashar al-Assad, a member of the heterodox Alawite sect.

Jonas Parello-Plesner, a Danish diplomat at the Washington-based Hudson Institute, said that Trump's move "would make Russia decisively the outside power-broker in Syria."

Israeli Prime Minister Benjamin Netanyahu quickly pledged to "defend ourselves" in Syria, an allusion to Israeli strikes on targets of Iran and its Lebanese-based ally Hezbollah.

- Risks for Europe -

While the Islamic State group has lost virtually all of its territory in Syria, it is believed to have thousands of supporters who may carry out attacks overseas, often blending into local populations in Europe.

The US withdrawal leaves France, which still has a small contingent of special operations troops in Syria, and Britain, which according to media reports has quietly deployed a number of soldiers.

Former Belgian prime minister Guy Verhofstadt said a US withdrawal would be a victory for Russia, Iran, Turkey, Turkish proxies and the Syrian regime.

"Unsurprisingly, it leaves Europeans more vulnerable -- and shows how wrong it is that we do not have a defense force able to help stabilize our immediate neighborhood," he wrote on Twitter, amid French-led calls for a European-wide army separate from NATO.

- Effect on US politics -

Trump -- much like Obama, despite his distaste for his predecessor -- has called for an end to long-term foreign military interventions, which are costly and have limited support in the general public.

Trump's decision nonetheless was condemned both by the rival Democrats, who said he had not thought through his decision, and Republicans, who feared the geopolitical effects.

Republican Senator Lindsey Graham, usually a loyal supporter of Trump, charged that ISIS was not defeated and that a withdrawal would embolden Iran and abandon Kurdish allies.