The part of Syria that's now under attack by Turkey has been controlled by the Syrian Kurdish Democratic Union Party (PYD) and the armed Kurdish People's Protection Units (YPG), a defense force mostly made up of Kurds.
Now neighboring Turkey has launched a military operation to move the Kurds away from its border and the frequently targeted ethnic group is once again under attack.
How did it come to this? Here's what you need to know:
Who are the Kurds?
The Kurdish people are an ethnic minority group without an official state. Before World War I, Kurds lived a nomadic lifestyle until the breakup of the Ottoman Empire, which stripped them of their freedom and divided them across several nation states.
Today, there are an estimated 25-30 million Kurds, the majority living in a region that stretches across parts of Turkey, Iraq, Iran, Syria and Armenia.
Most Kurds are Sunni Muslims, but the Kurdish population has diverse cultural, social, religious and political traditions as well as a variety of dialects.
The Kurds have never achieved nation-state status, except in Iraq, where they have a regional government called Iraqi Kurdistan.
In some areas, Kurds have struggled to maintain their identity and continue to face discrimination and policies of persecution. In Turkey, for example, they're often referred to as "Mountain Turks," forbidden to wear traditional Kurdish outfits, speak their language or give their children some Kurdish names. They continue to face discrimination and policies of persecution.
Kurds are the largest ethnic minority in Turkey, making up roughly 20% of the population.
Where do Kurds live?
Today, Kurdistan is made up of five different regions: southeastern Turkey, northeastern Syria, northern Iraq, northwestern Iran and southwestern Armenia.
In the early 20th century, the Kurds began working toward the creation of homeland known as Kurdistan. In 1920, the Treaty of Sèvres -- one of a series of treaties that the Central Powers signed after their defeat in World War I -- outlined the dissolution of the Ottoman Empire and called for an autonomous Kurdistan.
Three years later, after the end of the war, Western allies dropped demands for an independent Kurdish state and the Kurdish region was divided among several countries.
What does Turkey have to say?
Turkish President Recep Tayyip Erdogan has always adopted a robust attitude against Kurdish nationalism. The President made it clear that his ultimate goal is to eliminate the Kurdistan Workers' Party (PKK), a Kurdish far-left militant and political organization based in Turkey and Iraq that fought the Turkish state for more than three decades.
This has built a deep divide between Turks and Kurdish Turks. In 2016, pro-Kurdish media outlets were closed down, more than 11,000 teachers were dismissed or suspended over alleged PKK connections and at least 24 government appointees replaced Kurdish mayors in the country.
Turkey has long been unhappy about the strong Kurdish presence in northeast Syria near the Turkish border. The country's military has already moved into portions of areas previously held by the Kurdish-led and US-backed Syrian Democratic Forces (SDF), but now their longtime plan to create a buffer zone in northern Syria is underway.
There are two goals: drive the Kurds away from their border, and use this area to resettle around 2 million Syrian refugees.
A cycle of betrayal?
After the US-led coalition expelled Saddam Hussein's Iraqi forces from Kuwait in 1991, US President George H.W. Bush urged Iraq's military and its people to overthrow the dictator. But when the Iraqi Kurds rose up against Hussein, they were given little support. Millions fled their homes as the Iraqi military slaughtered Kurds in their thousands.
During the US invasion of Iraq in 2003, the Kurds were once again enthusiastic allies. And when ISIS swept through Iraq, Kurdish fighters were crucial in the effort to push the terrorist group out. In Syria, the YPG is the predominant force in the US-backed Syrian Democratic Forces (SDF), and was the main ground force that pushed ISIS from its headquarters in Raqqa.
But in both cases, the support offered by the US faded as the political climate changed. In 2018, the US stood by as Iraqi forces drove the Kurds back from territory they had gained battling ISIS. Now, as US forces in the area withdraw, the Kurdish presence in northern Syria is being attacked by Turkey.
That history with the United States has led some Kurds to feel betrayed by their most powerful ally. "The message here to all of our friends in the Middle East is that the United States is unreliable," former US ambassador to NATO Nicholas Burns told CNN.
Why is Turkey attacking them now?
In January, President Trump said the United States would begin withdrawing troops from Syria and "slowly" send them back home. At the time Syrian Kurds feared Turkey would use the withdrawal as an opportunity to launch an assault.
And after Trump's sudden announcement Sunday night, these fears were realized.
"Turkey will soon be moving forward with its long-planned operation into Northern Syria," a White House statement said. "The United States Armed Forces will not support or be involved in the operation, and United States forces, having defeated the ISIS territorial 'Caliphate,' will no longer be in the immediate area."
As of last month, the United States said about 1,000 US troops were operating in northeastern Syria. This move leaves Syrian Kurds at the mercy of an operation aimed to clear them out and away from the Turkish border.
"#OperationPeaceSpring will neutralize terror threats against Turkey and lead to the establishment of a safe zone, facilitating the return of Syrian refugees to their homes," Erdogan tweeted Wednesday. "We will preserve Syria's territorial integrity and liberate local communities from terrorists."
How will attacking the Kurds risk the return of ISIS?
The SDF, a coalition of Kurdish and Arab soldiers backed by US, British and French special forces, defeated ISIS and liberated eastern Syria in March. The SDF said it lost 11,000 "forces, leaders, and fighters" while battling ISIS.
While ISIS holds virtually no more territory, the United States warns there are still tens of thousands of ISIS fighters in hiding in both Iraq and Syria.
Thousands of ISIS fighters who were captured during major battles against the terrorist group are being held by the SDF. Somehow, all these fighters may be handed over to Turkey.
As many Syrian Kurds prepare to fight and others run in the opposite direction, thousands of terrorists and two holding facilities containing displaced ISIS members and ISIS survivors could be left ungoverned. This poses a serious risk as ISIS will have an opportunity to reestablish itself.
This story has been updated.
CNN's Alessandria Masi and Pete Burn contributed to this report.