Yet again people around the world are recoiling in horror and outrage over images coming out of Syria, this time after dozens of people, including children, were reported killed by a suspected chemical attack in the rebel-held province of Idlib in northwest Syria. It’s not the first deadly attack by Syrian President Bashar al-Assad’s forces or the Russian and Iranian paramilitaries helping him. Throughout the years this conflict has persisted, the cry of ‘never again’ has echoed in the corridors of power and across the Internet.

On Tuesday U.S. President Donald Trump, like many other world leaders, condemned the attack, calling it “reprehensible and cannot be ignored by the civilized world.” He largely cast the blame for Assad’s emboldened posture on President Barack Obama’s management of the crisis, reminding everyone that Obama had established a ‘red line’ that chemical weapons would invoke the use of military action by the U.S., but as Trump pointed out, “did nothing,” (or close to it) afterwards.

The 2012 incident was one of many the world has witnessed. Other headlines have provoked the same horror and sadness. Here are some of the events in the Syria conflict that have triggered headlines and outrage around the world, as the death toll rises to more than 400,000 and millions of Syrians have fled the country.

The Houla massacre took place on May 25, 2012 in the town of Talduo, northwest Syria. Reports at the time said at least 108 people were killed in the carnage carried out by Syrian forces and the pro-Assad Alawite paramilitary militias, known as Shabiha, who shelled the Sunni-dominated town, according to the BBC. It was one of the first incidents to drew global and Arab attention to the crisis in Syria and became another symbol of the regime’s brutality. Hashtags in Arabic for the #HoulaMassacre and #HoulaChildrenMassacre reached more than 300,000 posts.

Then U.S. ambassador to the United Nations, Susan Rice, suggested military intervention might be the only remaining option, but the then-Defense Secretary Leon Panetta said he didn’t see the U.S. taking military action without the backing of the U.N. Security Council.

"@adel0503: شاهد أهلنا في سوريا يشيعون ضحايا مجزرة الحولة في مقبرة جماعية، أكثرهم أطفال … pic.twitter.com/SnTesfvV — Norah (@Norah_Nasser) May 29, 2012

Watch our people in Syria burying the victims of Houla massacre in a mass grave, most of them are children.

The massacre in Ghouta, August 2013, was the deadliest chemical attack in Syria ever. Up to 1,500 people were said to have been killed after rockets with the chemical agent sarin hit the Damascus suburb and prompted then-President Obama to ask Congress to authorize military action against Syria in 2013. Syria agreed to a Russian-American proposal to give up control of its chemical weapons, and Obama gave up on a military campaign against Assad. U.N.-OPCW investigators found, however, that Assad continued to use chlorine, widely available and hard to trace, in so-called barrel bombs dropped from helicopters, according to Reuters.

Obama’s red line on the chemical in Syria turned to a green light to continue killing.

The infamous photographs three-year-old Aylan Kurdi triggered global heartbreak in September 2015. The body of the toddler was found lying face down in the surf not far from Turkey’s tony resort town of Bodrum. His fate became one of the most tragic symbols of Europe’s migrant crisis, capturing the desperation, risk and tragedy afflicting millions fleeing war-torn regions in Africa and the Middle East.

A young Syrian boy lies in the surf near Bodrum, #Turkey #Syria pic.twitter.com/NrEf30zg3e — Wajd Bouabdallah (@tounsiahourra) September 2, 2015

Five-year-old Omran Daqneesh, filmed in August 2016 sitting barefoot in an ambulance, covered in dust and bleeding from the head, staring ahead blankly after surviving a Russian or Syrian airstrike in the Qaterji neighborhood of eastern Aleppo. Assad claimed the pictures showing the boy pulled out of the rubble were faked, but Danqeesh and the people of Aleppo became another rallying cry for the world to act to stop Syria’s war, again to little avail.