What you need to know about who’s fighting in Syria and what the US withdrawal means

What started as a civil war in Syria has mutated into a regional proxy conflict.

For over four years, the U.S. armed and fought with Syrian Kurdish forces who served as America's foot soldiers against the Islamic State terror group. But with President Donald Trump pulling American troops out of Syria, those Kurdish forces announced on Sunday that they have a new partner: Syrian leader Bashar al-Assad.

Alliances in Syria have evolved throughout the country's eight-and-a-half year-old war, but the withdrawal of U.S. forces and the Kurdish alignment with Assad is one of the most dramatic changes that heralds a new chapter in what has already been an unending, horrific conflict.

It gives Assad -- the strongman president who tried to put down an initial rebellion in 2011 with brutal force and has bombed, gassed, tortured, and jailed his own people in the ensuing civil war -- effective control over Kurdish-held territory in the northeast.

That leaves just one pocket of opposition-held territory in the country's northwest, which Assad has moved in recent months to conquer militarily, with air power from his sponsor Russia. But Turkey has troops stationed there as it backs opposition groups, including some with ties to al-Qaeda and other Islamist terrorist groups -- setting up a showdown of Turkish and opposition forces against Syrian and now Kurdish forces.

Here's a look at the key players involved in Syria in what started as a civil war and has mutated into a regional proxy conflict.

Which Kurds?

The Kurds are an ethnic group that have historically inhabited the highlands of Iraq, Syria, Turkey, Iran and Armenia, united across borders by culture and language. They practice different religions, but the majority are Sunni Muslims. They never have had their own country, with each of those nations struggling, to varying degrees, against Kurdish independence movements.

In Turkey, one of those independence movements -- the Kurdistan Workers' Party, or PKK -- is designated a terrorist organization, including by the U.S. since 1997. For decades, Turkey has sought to squash the PKK and experienced PKK terror attacks, which the U.S. has supported its NATO ally against.

But across Turkey's southern border in Syria, the U.S. partnered with Syrian Kurdish forces to fight ISIS. The People's Protection Units, or YPG, were the main fighting forces in the Syrian Democratic Forces, or SDF, which served as the foot soldiers for the U.S. and its global coalition to defeat ISIS. It was a partnership that always angered Turkey, who makes no distinction between the PKK and YPG. But U.S. officials said it was the only option because the U.S. and other countries were unwilling to send troops in and Turkey was unwilling or unable to effectively fight ISIS.

The departing Americans

The U.S. had up to 2,500 troops on the ground in Syria, which were so closely partnering with SDF troops, that the Kurds directly called in U.S. air strikes as they fought block by block to retake cities and towns from ISIS. They lost approximately 11,000 fighters -- male and female -- in that offensive. In the months since the ISIS "caliphate" fell, the U.S. continued to work with the SDF to stabilize liberated areas and eliminate remaining ISIS cells.

But last December, after a phone call with Turkish President Recep Tayyip Erdogan, Trump ordered all U.S. forces out of Syria. That sudden decision led to the resignation of his Defense Secretary James Mattis and special envoy for ISIS Brett McGurk, although eventually it was slowed down as U.S. officials said they wanted to ensure it was a safe and deliberate process.

There are approximately 1,000 American troops in the country now, but nearly all of them will be departing Syria soon, according to the Pentagon. Their departure, announced after another Trump-Erdogan call on Oct. 6, has allowed Turkish forces and their opposition allies to move in against the Kurdish fighters.

Turkey and its rebel allies

That offensive began last week, prompting international condemnation and U.S. sanctions. Turkey has conducted air and artillery strikes against Syrian Kurdish forces as its proxy forces have moved into Kurdish-held territory.

The main fighting force there is the National Army, formerly known as the Free Syrian Army -- a rebel force of Syrian military defectors and ordinary citizens who were armed and trained by the U.S. until Trump came to office and ended the program. They have pushed back into one last pocket in Syria's northwest, where they are boosted by Turkish troops.

But other groups under this opposition umbrella have ties to al-Qaeda, the most powerful of which is Hayat Tahrir al Sham, or HTS, the latest incarnation of the al-Nusra Front, which was al-Qaeda's Syrian affiliate.

The Kurds' new ally, Assad

These rebel groups have been Assad's primary target as he moves to retake the last territory out of his control, the Idlib province. In recent months, his troops and allies -- Iran and Hezbollah, the Lebanese paramilitary organization -- have skirmished with rebels there as Russian air strikes bombed cities, including hitting civilian targets. A Turkish convoy was hit in an August air strike, heightening tensions.

After the U.S. withdrew its troops, Kurdish forces reached an agreement with Assad on Sunday. Government troops were already moving into Kurdish-controlled territory for the first time in years. That could block Turkish forces from moving further south into Kurdish-controlled terrain, or set up more direct clashes between two militaries.

Trump said in a tweet on Monday that the U.S. considers Assad "our enemy," and there are extensive sanctions against his regime, Iran and Russia for supporting it, and those that do business with it. But it's unclear if the Kurds will be sanctioned for partnering with him now too. The State Department has not responded to questions about that.