The Syrian refugee crisis crashing against Europe's shores has been, in many ways, a wake-up call for the West: that it has a responsibility toward refugees, that there are dire and growing crises in the world, that checkbook humanitarianism won't cut it anymore. And it has forced us to confront a truth we have known but struggled to avoid for years: Syria's civil war is one of the greatest catastrophes of our lifetimes.

How, many are naturally wondering this week, did this catastrophe come to be? How did it become so bad? Colleague Zack Beauchamp offers a brief, simple explanation of Syria's war. But it is worth dwelling in particular on how this war began, and how it spiraled from a few Arab Spring protests into a war so horrific it has killed 250,000 and displaced half of Syria's population.

Iyad al-Baghdadi, an Arab Spring activist now living in Norway, recounted the war's origins in a series of tweets on Saturday. His timeline, four years after the events he describes, is difficult to read, looking back on the courage of those Syrian protesters, knowing what Syrian leader Bashar al-Assad and the world had in store for them. Baghdadi's tweets (reproduced here with his generous permission) are a helpful primer, a reminder of the evils of the Assad regime, and an implicit but devastating reminder of how the world has over and over failed Syria's people:

December 2010/January 2011: The Tunisian and then the Egyptian revolutions kick off the Arab Spring. Tunisian dictator ousted. — Iyad El-Baghdadi (@iyad_elbaghdadi) September 5, 2015

Late Jan 2011: Syrian youths try to organize protests/revolution for Feb 5 2011 (it doesn't work). #Syria — Iyad El-Baghdadi (@iyad_elbaghdadi) September 5, 2015

31 Jan 2011: In light of events in Tunisia & Egypt, Bashar al Assad says "it's time for reform" (then does nothing). #Syria — Iyad El-Baghdadi (@iyad_elbaghdadi) September 5, 2015

Feb 2, 2011: 20 Syrian youths hold a candlelight vigil for Egypt's revolution. They're attacked & beaten up by thugs. #Syria — Iyad El-Baghdadi (@iyad_elbaghdadi) September 5, 2015

Feb 11, 2011: Mubarak is ousted in Egypt. Every Arab tyrant's freak out level goes to "extreme", Bashar included. — Iyad El-Baghdadi (@iyad_elbaghdadi) September 5, 2015

Feb 15, 2011: Teenage blogger Tal al Mallouhi sentenced to 5 years in prison for "espionage" in closed hearing. #Syria — Iyad El-Baghdadi (@iyad_elbaghdadi) September 5, 2015

Feb 17, 2011: Small protest in Damascus protesting police brutality and corruption. Look at the numbers. http://t.co/YT3ZtmKioI — Iyad El-Baghdadi (@iyad_elbaghdadi) September 5, 2015

Feb 20, 2011: Blogger Ahmad Abulkheir arrested for praising Tunisian revolution and posting anti-censorship information. #Syria — Iyad El-Baghdadi (@iyad_elbaghdadi) September 5, 2015

Feb 23, 2011: Rally in solidarity with the Libyan revolution is violently dispersed. #Syria — Iyad El-Baghdadi (@iyad_elbaghdadi) September 5, 2015

Mar 6, 2011: Libyan rebels shoot down two Gaddafi fighter jets. Pilots reported to have had Syrian passports. #Syria — Iyad El-Baghdadi (@iyad_elbaghdadi) September 5, 2015

Mar 15, 2011: "Official" start of the revolution. Pre-planned rallies across #Syria: Damascus, Aleppo, Daraa, Homs, Banyas. — Iyad El-Baghdadi (@iyad_elbaghdadi) September 5, 2015

Mar 18, 2011: Violent clamp down leads protests to spread & escalate across Syria - actual start date of the revolution. — Iyad El-Baghdadi (@iyad_elbaghdadi) September 5, 2015

Mar 20, 2011: Baath party headquarters in Daraa torched. Protests spread and intensify. #Syria — Iyad El-Baghdadi (@iyad_elbaghdadi) September 5, 2015

Mar 22, 2011: Tense stand off at Omari mosque in Daraa. Eventually 12 demonstrators die and 200 are injured. #Syria — Iyad El-Baghdadi (@iyad_elbaghdadi) September 5, 2015

Mar 23, 2011: Regime already describes peaceful protests (referred to as a "case") as a foreign conspiracy carried out by Alqaeda. #Syria — Iyad El-Baghdadi (@iyad_elbaghdadi) September 5, 2015

Mar 30, 2011: Assad gives a speech to his parliament. Talks about emergency law and allowing multiple parties. Clamp down goes on. #Syria — Iyad El-Baghdadi (@iyad_elbaghdadi) September 5, 2015

Mar 31, 2011: Assad forms a "committee" to study lifting emergency law, which has been in effect since 1962. #Syria — Iyad El-Baghdadi (@iyad_elbaghdadi) September 5, 2015

Early April 2011: Protests continue across #Syria. Peaceful, met with force by the regime. Here's one from Homs: http://t.co/L1AyPzhAA0 — Iyad El-Baghdadi (@iyad_elbaghdadi) September 5, 2015

April 7, 2011: Kurds protest in Qamishli despite offer of citizenship by Assad. http://t.co/4BzThgbgQJ #Syria — Iyad El-Baghdadi (@iyad_elbaghdadi) September 5, 2015

April 8, 2011: 23 reported dead after live gunfire used against protests in Daraa. Largest demonstrations so far. #Syria — Iyad El-Baghdadi (@iyad_elbaghdadi) September 5, 2015

April 12, 2011: Regime starts a military operation in city of Banias after these demonstrations: http://t.co/ND3Ky3mZyU — Iyad El-Baghdadi (@iyad_elbaghdadi) September 5, 2015

April 22, 2011: Protests reach Midan in the heart of Damascus. Overall death toll on this day 80+. #Syria http://t.co/f8UVyLWw1M — Iyad El-Baghdadi (@iyad_elbaghdadi) September 5, 2015

April 25, 2011: Full scale military operation by Assad's army in Daraa. Rumors of defections. City is blockaded. #Syria — Iyad El-Baghdadi (@iyad_elbaghdadi) September 5, 2015

May & June 2011: Daily unarmed protest across #Syria calling upon Bashar to leave. Security clamp down continues. http://t.co/JpNeiH0Vy0 — Iyad El-Baghdadi (@iyad_elbaghdadi) September 5, 2015

9 June 2011: Lt. Col. Harmoush among first confirmed defections. Video says him & others will "protect the peaceful protests". #Syria — Iyad El-Baghdadi (@iyad_elbaghdadi) September 5, 2015

Mid June 2011: Regime in full scale military operation in the north after defections. Revolution begins to adopt the pre-Baath flag. #Syria — Iyad El-Baghdadi (@iyad_elbaghdadi) September 5, 2015

Warning: the link in the tweet below links to an extremely graphic image.

July 8, 2011: Body of Ibrahim Qashoush, who wrote chants for the protests, found dumped in river. Larynx ripped out. http://t.co/pg70mkcMoI — Iyad El-Baghdadi (@iyad_elbaghdadi) September 5, 2015

July 19, 2011: Demonstrators have not backed down anywhere in #Syria. Tanks deployed against protesters in Homs. — Iyad El-Baghdadi (@iyad_elbaghdadi) September 5, 2015

July 29, 2011: The Free Syrian Army (FSA) is declared. — Iyad El-Baghdadi (@iyad_elbaghdadi) September 5, 2015

Foreign policy communities in Washington and elsewhere have debated ever since then, and will probably debate for the rest of our lives, what, if anything, the world could have done differently to prevent this catastrophe. I can't claim to have an answer; I'm not sure anyone can. Baghdadi offers some thoughts that are worth reflecting on as the repercussion of 2011 now wash onto European shores:

The Syrian revolution did not start sectarian, and sectarianism isn't its driving impetus. Sectarianism was imposed upon it much later. — Iyad El-Baghdadi (@iyad_elbaghdadi) September 5, 2015

Blaming "everyone" is a cop out. Four years ago Assad assaulted a peaceful revolution and transformed it into a a bloody civil war. — Iyad El-Baghdadi (@iyad_elbaghdadi) September 5, 2015

The Syrian catastrophe was very preventable, if the world's red line was "killing protesters" rather than "drowned refugee toddlers". — Iyad El-Baghdadi (@iyad_elbaghdadi) September 5, 2015

And yes, they still want their liberty. Caption: "2011//12/13/14/15... and we still want our liberty". Aleppo. pic.twitter.com/voM0NU7VzH — Iyad El-Baghdadi (@iyad_elbaghdadi) September 5, 2015