 After more than three years of civil war, Syria is home to the world's largest, most complex, and dangerous human crisis. More than 160,000 people have died. The Assad regime is the driver. It violently targets civilians. Armed militant groups are guilty as well. More than 9.5 million Syrians have been forced from their homes, nearly half the population. This includes three million refugees who have fled to Lebanon, Jordan, Turkey, and Iraq, overburdening these neighbors and threatening instability. There is no political settlement or military victory in sight. As the human crisis worsens, Syria has also become a haven for armed Islamist extremists. This has opened the door for ISIS, the Islamic State of Iraq and Syria, to launch its armed invasion of Iraq. Each month, 100,000 new Syrian refugees spill into the region. There are 6 million people displaced inside Syria. They struggle to find security, medical care, and housing, and half are not reachable. A generation of children are without schooling, vulnerable, traumatized, and ill. The Assad regime blocks safe delivery of humanitarian relief in defiance of the UN Security Council, hundreds of armed factions impede relief. But countless Syrians and international personnel risk their lives daily to deliver assistance. Many have been killed or wounded. The Assad regime also routinely targets health workers and facilities. Many medical professionals have been killed or detained. Half of Syria's doctors have left the country, and a large portion of hospitals and clinics have stopped functioning. The public health consequences are profound. Disrupted immunizations for polio, measles, and meningitis. A breakdown of care for diabetes, cancer, and hypertension. Complications in childbirth. And Syria is now an exporter of polio, contributing to a global polio emergency. The United States has generously contributed more than $2 billion in relief, but the world's response still falls far short, and there is a danger of fatigue. Syria's colossal human disaster has triggered a crisis of conscience. There are no easy answers, just questions about what can be done, short of a resolution of the war itself. Armed the moderate opposition. Expand cross-border delivery, ignoring the Assad regime's objections. Pursue humanitarian interventions, including no-fly zones. Prosecute the Assad regime for war crimes. Expand assistance to stabilize Lebanon and Jordan. How to cope with the humanitarian fallout from an ISIS proto-state that is violently anti-Western, and now controls large parts of Syria and Iraq. As the debate unfolds, we will struggle for many years to come with the legacy of Syria's human crisis. Mass suffering, widespread societal damage, a possible lost generation, and grave new security threats affecting Syria and its neighbors alike.