The deepest impact of the Syrian civil war, now more than three and a half years old, has been the displacement of millions of people amid the conflict. More than a quarter of all Syrians have been forced to flee their homes, placing immense burdens on neighboring countries providing sanctuary and leaving an indelible trauma on a whole generation of the country's population. In neighboring Iraq, the onslaught of the Islamic State this year has led to the displacement of nearly 2 million.

So it's a source of continuing alarm and tragedy that the most dramatic Syrian refugee exodus is taking place now -- as many as 150,000 Syrian Kurds have fled the advance of Islamic State fighters and poured across the border into Turkey over the past three days. Ankara has locked down parts of the border to deal with the influx. The situation is stark enough, but further complicated by two linked factors: Turkey's own controversial role in the larger fight against the Islamic State and its troubled dealings with Kurdish factions, both in Turkey and in surrounding countries. Here's what you need to know.

What's happening?

As my colleagues reported Monday, an Islamic State offensive in recent weeks has overrun a number of Syrian Kurdish villages near the city of Kobane. Activists and refugees have reported that the extremists are carrying out random slaughter, including beheadings, of Kurds whom they capture. While many Kurds are Muslim, the Islamic State views them largely as secular apostates.

Turkish forces have dispersed protesters at the border with Syria amid an escalating refugee crisis. More than 130,000 Syrian Kurds have crossed into Turkey in just three days. (Reuters)

The bulk of the refugees arriving in Turkey are from Kobane, perched along the Turkish border. The town has yet to fall into the Islamic State's hands: Fighters from the People's Protection Units, the main armed Syrian Kurdish faction, also known by the acronym YPG, are battling the jihadists along the road to the city. "Taking Kobane would give the Islamic State control of a large swath of the Syrian-Turkish frontier and another potential route for Islamic State recruits," report The Post's Rebecca Collard and Brian Murphy.

"I don’t think in the last three and a half years we have seen 100,000 cross in two days," said Carol Batchelor, a representative for the United Nations refugee agency in Turkey, over the weekend to Reuters. "It could well go again into the hundreds of thousands. We need assistance for core, lifesaving support."

Who are the Kurds?

The Kurds are among the most high-profile ethnicities in the world to be without their own nation-state. Kurdish communities are scattered through northern and eastern Syria, Iraq, Turkey and Iran. The borders here have often been porous and an alphabet soup of Kurdish militias from all four countries have taken part in the ground war against the Islamic State's positions in Iraq. YPG units this summer led the rescue of thousands of Yazidis stranded on Iraq's arid Mount Sinjar, where they had been chased by Islamic State fighters bent on their eradication.

The campaign to push back the Islamic State's advances in Iraq has empowered the Kurdish Regional Government, which administers a largely autonomous Kurdish region in Iraq's north. Its units, known as the pesh merga, have received aid and arms from the U.S. and are being supported by American airstrikes. Many in Washington look to the Kurds in Irbil as a more reliable partner in the region than the oft-divisive, dysfunctional Iraqi government in Baghdad.

The YPG, meanwhile, has spent much of the Syrian civil war battling Islamist forces. In keeping with other Kurdish militias, the YPG likes to flaunt its secularism, boasting units of female fighters who fight in the open alongside the men. The militia has institutional links to the Kurdistan Worker's Party, or PKK, a Kurdish guerrilla force that waged a decades-long insurgency in Turkey. Kurds comprise the largest ethnic minority in Turkey and their language and culture was suppressed through decades of authoritarian, Turkish nationalist rule. The YPG is the military wing of the PYD, a Syrian Kurdish political organization that is a direct offshoot of the PKK.

Turkey and the U.S. State department both consider the PKK a "terrorist" group, even though the PKK, whose fighters relocated to camps in Iraqi Kurdistan as part of a peace deal with Ankara, has been instrumental in the fight against the Islamic State.

Turkey's problem

Kurds on both sides of the Syrian-Kurdish border were furious with Turkey's closing of border crossings, which both impeded the passage of terrified refugees and also prevented Kurdish fighters on the Turkish side from crossing the border to aid in the fight against the Islamic State. "Turkey is preventing, not only PKK, but all Kurdish men from entering Syria,” Redur Xelil, a YPG spokesman, told The Post's Collard. He raised a longstanding claim by many Syrian Kurds: "The reality is that Turkey is siding with" the Islamic State.

Turkish officials reject such criticism, and insist that they have the most to lose with the Islamic State on its borders and the burden of coping with millions of Syrian refugees. They say they're cracking down on the clandestine networks that ferry foreign jihadists into Syria and Islamic State-controlled oil out.

But many remain unconvinced. Turkey has yet to join a coalition of Arab countries committed to supporting the U.S.-led campaign against the Islamic State, and it's unlikely that the NATO member will engage the jihadists militarily.

While Ankara has warmed to the regional Kurdish government in Iraqi Kurdistan, it looks far more skeptically at the YPG and its affiliates in Syria, which it sees as a PKK proxy, and would likely not want such factions to gain significant ground along its border. As the refugee crisis escalated over the weekend, the PKK called for Kurds in Turkey to aid their brethren across the border. Amid patriotic songs on the radio, according to Reuters, the Kurdish outfit proclaimed that "supporting this heroic resistance" in the town of Kobane was a "debt of honor."