On the last day of September, Russia officially began bombing targets in Syria. It said it was bombing ISIS — but it was really targeting opposition groups that are fighting Bashar al-Assad's regime (incidentally, they're also fighting ISIS).

This has come after a month in which Syria's war, raging since 2011, was at the center of global attention. The war has included the crisis of 4 million refugees, symbolized by Syrian toddler Aylan Kurdi, whose body washed up on the shores of Turkey in early September after his family attempted to make their way to Europe.

The roots of the Syrian civil war go back years before fighting began in 2011. Since it broke out, it has gone through several dramatic changes, each of which has made the geopolitics surrounding the conflict more fraught — and life worse for the Syrian civilians who suffer most.

So what follows is a (relatively brief) guide to the events of the Syrian war, as well as the events that presaged it. The war has killed at least 250,000 and forced over half of the population from its homes, and is still raging. It helps to start the timeline well before 2011, going back to the country's postcolonial roots and the 1976 fighting that one scholar calls the "first round" of today's war.

1923–1946: French imperialism and the ingredients for sectarianism

Here is a key historical fact for understanding the Syrian crisis: Syria's borders were invented in large part by French colonialists, forcing together several disparate groups.

After World War I, the Ottoman Empire collapsed, and out of its ruins France took control, roughly in 1920, of a stretch of Ottoman territory on the eastern Mediterranean, today known as Syria and Lebanon. This territory was, and remains, quite ethnically and religiously diverse.

There are lots of countries whose borders were imposed by European imperialists that manage to do fine, but the point is that French colonialism set up modern-day Syria in a way that contributed to tension between ethnic and religious groups, which eventually became important for today's war.

One religious minority group, Alawite Shia Muslims, in particular saw French colonialism as an opportunity. Long persecuted as "heretics" by the much larger population of Sunni Muslims, some Alawites joined up with French colonial authorities. In particular, they joined the newly constructed Syrian military — a development the French welcomed as a means of cementing their power. European colonialists often promoted minority groups that would rely on them to maintain power, so the relationship was symbiotic.

The French eventually left, but the borders stayed. In 1963, the Syrian military took power, and the government quickly became dominated by Alawites.

"When Hafez al-Assad came to power in 1970, virtually all the top power brokers in Syria were Alawites," Glenn Robinson, an associate professor at the Naval Postgraduate School, writes.

This created a dangerously precarious political situation. The Alawites saw (and continue to see) maintaining a sectarian government as their best hope for securing themselves from persecution and even massacre. But the Sunni majority was blocked from meaningful political power and angry about it. This could only last for so long.

1982: The Hama massacre that set Assad regime rules for uprisings

The best way to see Syria's civil war isn't as something that started, out of nowhere, in 2011. Rather, think of it as a resumption of what Robinson calls "the first round" of Syria's civil war — which began in 1976 and ended with a shockingly brutal massacre in the city of Hama.

That year, Hafez al-Assad had Syrian forces intervene in Lebanon's civil war, on behalf of Lebanese Christian groups who were fighting Muslim groups. The Muslim Brotherhood and many other Syrian Sunnis saw this as heresy, proof that the Assad regime needed to go. They launched a low-intensity civil war, which went on for six years. To counter them, the Alawite regime courted allies among privileged Sunnis and the Christian minority.

Assad finally ended the war in a particularly brutal fashion: In 1982, he nearly leveled the city of Hama, where the opposition was strongest, slaughtering thousands of civilians in an indiscriminate barrage. The regime learned from this experience that mass violence was the smart response to unrest — a lesson that was applied particularly brutally in 2011.

But that was the wrong conclusion to take. Hama didn't solve the real causes of Syria's strife: authoritarianism, a fundamentally unequal balance of power, and threats between demographic groups. It merely put off a reckoning.

"The root issues and the competing sides have been the same" in both 1976 and 2011, Robinson writes. "A minority based regime, allied with other minorities along with privileged elements from the majority population, ruling over a poor and often dysfunctional state that does not tolerate dissenters."

March 2011: The Deraa protest that began the uprising

Uprisings tend to happen in waves. It's no accident, then, that the Syrian protests began in March 2011: They came on the heels of huge, successful Arab Spring uprisings in Tunisia and Egypt. But promise of peaceful revolution quickly gave way to a violent, bloody crackdown.

On March 18, Syrians in the southern city of Deraa took to the streets to protest the cruel treatment of several anti-regime youth. Security forces opened fire on the protestors, killing four.

Bashar al-Assad, Hafez's son and Syria's president since 2000, was applying the lessons he's learned from the Hama massacre: Kill first and ask questions later.

"The lesson of Hama must have been at the front of the mind of every member of the Assad regime" when protests began in 2011, William Polk, a longtime Syria expert at the University of Chicago, wrote in the Atlantic. "Failure to act decisively, Hama had shown, inevitably led to insurrection. Compromise could come only after order was assured."

At first, it seemed like the response backfired. The killing at Deraa sparked nationwide protests, which is why the Deraa crackdown today is widely acknowledged as the beginning of the revolution. These protests initially had no sectarian cast or agenda, and grew large enough to threaten the very foundations of the Syrian government.

The Assad regime continued to respond the only way it knew how — with force, killing more at subsequent protests. This was part of a deliberate strategy to turn a nonviolent conflict into a war.

"It was very much a strategic decision that the regime made, to militarize the conflict right away," Robinson told me in a phone conversation. "I think, in their mind and correctly, if this becomes a political battle where populations matter, the regime probably only has support of a third of the country. ... If this becomes a political contestation, the opposition has the numbers."

And that gruesome strategy is really at the heart of the country's current problems.

July 2011: The Free Syrian Army is founded

Perhaps inevitably, the Assad regime's routine pattern of slaughtering the opposition caused protesters to take up arms. The core of the conflict quickly became the fight between anti-regime rebels and the Syrian government — and remains so today.

The Free Syrian Army was the first major rebel military group to organize. It functioned as an umbrella group of different forces, drawn largely from defectors from Assad's military. "The regime was putting into battle only those units that it trusted," Robinson said. "Arab Sunni units that they did not, they just kept them in the barracks. And it's from those units that you got some defectors."

Early in the conflict, the Free Syrian Army seized a number of regime military bases, including the military hardware present there. "That's how they began to fight as a conventional force," Daveed Gartenstein-Ross, a senior fellow at the Foundation for the Defense of Democracies, told me.

As 2011 dragged on, the protester-led dynamic of the early revolution was gradually overtaken by outright fighting. By January 2012, the Syrian uprising had become a civil war.

August 2011: An Iraqi group arrives in Syria — and later becomes ISIS

During the Iraq War, one of the worst of several Sunni extremist groups was al-Qaeda in Iraq. The group was so awful that many Sunni Iraqis turned against it, helping to largely (though not completely) defeat the group by 2007 or so.

But by 2011, al-Qaeda in Iraq had begun rebuilding. And it saw the growing conflict in neighboring Syria as an opportunity to gain weapons, bases, and recruits.

In August 2011, AQI leader Abu Bakr al-Baghdadi sent a top deputy, Abu Mohammad al-Joulani, to Syria. His goal was to set up a new branch of the extremist organization in the country. Joulani succeeded, establishing Jabhat al-Nusra (about which we'll talk more in a second). Years later, the franchise would divide in two after AQI changed its name to the Islamic State in Iraq and Syria (ISIS) and asserted total control over Nusra. Some fighters pledged loyalty to al-Qaeda's central leadership, while others defected to ISIS.

This was all far off in mid-2011. But the fact that ISIS had operatives in Iraq as early as August 2011 illustrates how quickly it recognized that Syria was an opportunity — and just how deep its roots in the country go.

October 2011: In the wake of Libya, Russia vetoes a UN Security Council resolution condemning Assad

As Assad's crackdown worsened, international condemnation grew. In October, the UN Security Council considered a draft resolution condemning Assad's crimes — not intervening, not calling for a referral to the International Criminal Court, just condemning. Russia and China vetoed it. When another draft resolution was proposed in February 2012, they vetoed that one too.

This was mainly a Russian initiative. "The perception amongst diplomats in New York was that Beijing was [vetoing] out of solidarity with Moscow rather than commitment to Damascus," Simon Adams, the director of the Global Centre for the Responsibility to Protect, writes in a report on the UN and Syria.

This was part of Russia's long-running policy of providing Assad diplomatic cover from the world's outrage, no matter what he did. Moscow had been backing Assad in his war essentially since protests began. In 2011 alone, Russia sold nearly $1 billion in arms to the regime.

Ties between the two countries go all the way back to the Cold War: According to one scholar, the Soviets "essentially built" the modern Syrian military in the 1960s. Continued support for the Assad government yielded the USSR its most reliable ally and proxy in the Middle East.

Today, Syria remains one of Russia's few reliable allies outside of the former Soviet republics, a vestige of Moscow's former superpower status and a final military toehold in the Middle East. Russia maintains a valuable naval base today at Tartus, on Syria's Mediterranean coast, and has sold a number of surplus weapons to Assad. Between 2006 and 2010, 48 percent of Syrian arms imports came from Russia. So Russia has defended Assad to preserve its ally.

But the lengths that Russia has gone to protect Assad show there's more going on here. To understand that, you have to know the lessons that Russian President Vladimir Putin drew from another Arab Spring uprising that devolved into war: Libya.

In March 2011, Syria was still calm and Russia was formally ruled by Dmitri Medvedev (though Putin, as prime minister, remained powerful). Libya's uprising looked to be on the verge of terrible violence, and Western countries sought a UN Security Council resolution authorizing the NATO to intervene against government forces. Though Russia typically opposes such interventions, Medvedev abstained on the resolution, allowing it to go through. Russia — and particularly Putin — then watched with mounting horror as the intervention became a war to topple Qaddafi.

For Putin, what happened in Libya was the sum of many of his greatest fears: popular uprisings, collapsing authoritarian regimes, Western interventions, and extremism — all forces that he fears, in another context, could perhaps one day come to Moscow.

"[Putin] imagined the uprising in Libya as simply another step toward a revolution being orchestrated for Moscow," Steven Lee Myers writes in his book The New Tsar: the Rise and Reign of Vladimir Putin.

When protests spread to Syria, an actual Russian ally, Putin was determined not to let what had happened in Libya happen again. He leveraged Russia's military and diplomatic might to aid Assad in his war against his own people, including, a few months after the Libya intervention, by vetoing the UN Security Council resolution to condemn Assad.

And Russia would stay right alongside Assad every step of the way.

January 2012: The creation of Jabhat al-Nusra

The Syrian revolution did not begin as sectarian. But Sunni extremist groups quickly infected the opposition — with a little help from Bashar al-Assad. In amnesties issued between March and October 2011, he released a significant number (exact counts are hard to know) of extremists held in Syrian prisons.

Assad, it seems, hoped to sectarianize the conflict by boosting Sunni extremists in the opposition — and thus rallying Alawites and Christians to the regime's cause and deterring international intervention on the rebels' behalf.

It worked. According to Brookings's Charles Lister, many of Joulani's new recruits in Syria came from "individuals [recently] released in a series of amnesties granted by President Bashar al-Assad."

In January 2012, Jabhat al-Nusra — the new al-Qaeda franchise in Syria — announced its formation, with Joulani at its head. They were effective fighters and, by the end of that year, had linked up with many other anti-Assad rebels.

"When the US State Department designated Jabhat al-Nusra a terrorist organization on December 11, 2012," Lister writes, "the theme of that week's Friday protests across Syria was 'we are all Jabhat al-Nusra.'"

August 2012: Assad's first reported use of barrel bombs

As the conflict got worse, the death toll rose precipitously: As of March 2012, the UN estimated that around 9,000 Syrians had been killed in the fighting. By January 2013, the estimate was up to 60,000.

Much of that can be blamed on the Assad regime's vicious assaults on civilian-populated areas. Nothing symbolizes that brutality like the barrel bomb, whose first use was reported on August 22, 2012.

Barrel bombs are containers filled with explosives and sometimes metal, dropped from helicopters, often on civilian areas. Regime forces use them to cow the opposition, heedless of the civilian death toll.

"Assad’s indiscriminate use of barrel bombs deep in opposition-held territory means that for many there is no safe place to hide," Kenneth Roth, the executive director of Human Rights Watch, writes in the New York Times.

While barrel bombs are far from the only way that Assad's forces kill, the mere fact of their use shows how fully Assad has embraced indiscriminate violence and terror against civilians as a military tool.

Mid- to late 2012: Hezbollah invades Syria, a show of Iran's new role

By mid-2012, Assad was in serious trouble. He had lost effective control over much of the country, and many analysts believed a rebel victory was looking more likely.

Enter Iran.

Syria's alliance with Iran dates back to 1980, and is critical to Iran's regional strategy. It uses Syria to convey weapons and other goods to its proxies and allies, most notably Hezbollah in Lebanon and Hamas in Gaza. In return, Assad's regime gets military and political assistance from Tehran.

Iran thus saw Syria's revolt as a threat — Robinson described it as "a strategic issue of the first magnitude" for Tehran. In 2012, Iran responded by sending in Hezbollah to fight on Assad's side.

"In late 2012, US and Israeli officials received intelligence that the commander of the Iranian Quds Force, Qasem Soleimani, feared the Assad regime was in danger of being defeated by opposition forces," the Washington Institute for Near East Policy's Matthew Levitt writes. "Hezbollah would need to become involved in a much greater military capacity in Syria, or Soleimani argued the window of Iranian supplied advance weaponry directly to Hezbollah through Syria would close."

By that October, Hezbollah leader Hassan Nasrallah acknowledged that his fighters were on the ground (though he bizarrely denied sending them). Iran's support for Assad escalated from there. In early 2013, they were heavily involved in engagements like the al-Qusayr offensive in western Syria, where Hezbollah forces helped turn the tide in the government's favor.

It's impossible to say whether Iran's aid was the only thing that saved Assad from defeat back in 2012 and early 2013. But there's no doubt that Iran's assistance has been crucial to Assad — and a major reason the conflict has continued for so long.

March 2013: The Arab League endorses arming the rebels

Since the Iraq War, and maybe earlier, the oil-rich Arab states along the Persian Gulf — particularly Saudi Arabia, the largest and strongest — had been embroiled in a sort of Cold War with Iran. Both sides wish to steer the political course of the Middle East, and see the other as a fundamental threat to their security.

So when Assad began to teeter, the Gulf states saw an opportunity to unseat one of Iran's principal allies, and started sending arms to the Syrian rebels. Though this had gone on since at least 2012, the Arab League formalized it in March 2013, voting to give all members explicit permission to arm the Syrian opposition. In May of that year, the Financial Times reported that Qatar alone had given $3 billion in aid to the rebels.

But the problem here is that this huge cash infusion didn't all go to the good guys. Internal rivalries, particularly between Saudi Arabia and Qatar, caused the different Gulf states to pick different Syrian rebel groups to shower with cash. This was devastating, on two levels. First, it fractured and weakened the rebels: "Competition between their networks of rebel groups has been one of the major factors hindering the unification of the Syrian opposition," George Washington University's Marc Lynch writes.

Second, it helped extremists. Qatar in particular showered Jabhat al-Nusra with cash, helping the al-Qaeda franchise grow to become one of the most powerful anti-Assad forces in Syria. This, in turn, made it almost impossible for the United States or other interested powers to have any confidence that intervening against the regime would lead to a better Syria. Which is what Assad wanted all along.

April 2013: The birth of ISIS

In April 2013, something happened that would prove catastrophic for Syria: ISIS and al-Qaeda started breaking up.

ISIS leader Baghdadi had long had tensions with al-Qaeda's central leadership in Pakistan, and wanted to make sure he could control al-Qaeda's faction in Syria. But as Jabhat al-Nusra became stronger on the battlefield, it had begun to operate independently from Baghdadi.

On April 9, 2013, Baghdadi declared that Jabhat al-Nusra was part of al-Qaeda in Iraq — and that the new group would now be known as the Islamic State in Iraq and Syria, or ISIS. But Nusra's head, Joulani, refused to join up.

This left ISIS to "gradually emerge as an autonomous component within the Syrian conflict," according to Lister, by absorbing Nusra fighters and territory in northern and eastern Syria. In February 2014, ISIS was formally exiled from al-Qaeda, making it and Jabhat al-Nusra into enemies.

The competition between the two groups further radicalized the opposition, as there were now two powerful jihadist groups in Syria. And it gave ISIS the freedom to fully implement its hard-line, brutal ideology — and even incentivized it to out-extremist al-Qaeda in a competition for recruits and resources.

Assad, for his part, was perfectly happy to leave ISIS alone, in yet another example of his "encourage extremism" strategy. "ISIS almost never fought the Assad regime," Robinson says. "They were much more focused on fighting other opposition groups and gaining land their opponents had already acquired."

The full scale of the disaster wouldn't become clear until about a year later, when ISIS swept into northern Iraq and declared a caliphate in its territory in both countries. But the al-Qaeda/ISIS split was the beginning of a new, even darker period in Syria's war.

August 2013: The Ghouta chemical attack that shocked the world

On August 21, Assad's forces launched sarin gas — a horrifying and deadly chemical weapon — into the Damascus suburb of Ghouta, killing somewhere between 281 and 1,423 people, likely including hundreds of civilians.

It is perhaps the single most infamous event in the Syrian civil war. Morally, it symbolizes the depravity of Assad's strategy. Politically, it put the international community at a crossroads: Had Assad finally gone too far?

Since early in the uprising, the Obama administration had been calling for the Assad regime to go, but resisted any major efforts to help the rebels. But it had declared chemical weapons use to be a "red line": If Assad used them, the implication seemed to be, it could trigger an American military response.

But after Ghouta, Obama didn't seem to want to follow through. He submitted a plan for punitive airstrikes in Syria to Congress, where it was likely to fail.

Meanwhile, Russia was denying that the Syrian government had launched the attack. The Russians, of course, were still supporting Assad: Just that May, they had provided his forces with advanced S-300 anti-aircraft missiles, principally useful for deterring Western airstrikes.

That shows the Russians were still scared by the prospect of an anti-Assad intervention — a fear that explains why Russia got heavily involved in diplomacy after Ghouta. The Russians and the US made a deal: Assad would agree to give up his chemical weapons stockpile if the US backed off the threat of punitive airstrikes.

The chemical weapons deal was certainly a real accomplishment: It significantly lowered the risk of both chemical weapons use and of large deposits falling to ISIS or al-Qaeda. But Syria's rebels felt betrayed, and came to believe that the US would never fulfill its promises to help them. That's one of several reasons why subsequent US efforts to work with rebels have failed so dramatically.

September 2014: The US campaign against ISIS begins

In June 2014, ISIS swept northern Iraq, taking the country's second most populous city, Mosul. That August, it invaded Iraqi Kurdistan, which is a close US partner. This, together with the televised execution of two American journalists, prompted Obama to declare a plan to "degrade and ultimately destroy" ISIS on September 10.

This plan had to involve Syria, where ISIS has a substantial base. But there was a problem: ISIS can't be dislodged without a ground effort against them, but the US has no allies on the ground in Syria.

Obama created a program for training and equipping friendly rebels, but the plan has been totally botched in implementation. Roughly 54 US-trained Syrian rebels have been fielded, as of August — and about half were quickly killed or captured by Jabhat al-Nusra.

The program's failures point to the core contradiction in American policy. The rebels overwhelmingly see Assad as their key enemy: ISIS is a sideshow, and Nusra is often an ally. The United States, meanwhile, wants the rebels to focus on fighting ISIS and Nusra, and doesn't want to help them fight Assad. US and rebel priorities just don't line up. For another thing, "moderate" rebels have co-mingled with extremists for so long that there's no longer a clear line.

February to March 2015: The rise of the Kurds and rebel coalition Jaish al-Fatah

It's always too early for hope in Syria. But in early 2015, there were some signs that the bad guys were in trouble. Both Assad and ISIS lost ground, as this map of territorial changes in Iraq and Syria from January to June 2015 shows:

ISIS lost 9.4 percent of its total territory in both countries, while Assad lost 16 percent of his land in Syria, a staggering decline in just six months.

ISIS's defeats in Syria are primarily due to Kurdish advances in the country's north. For months, ISIS besieged the Kurdish town of Kobane — and was repulsed in February 2015, the group's first major defeat in Syria. Afterward, Kurdish forces, heavily backed by US airstrikes, went on the offensive. By late June, they were on the outskirts of ISIS's capital city of Raqqa.

Assad, meanwhile, has a lost a lot of ground — including some territory dangerously close to the coastal Alawite heartland. One key reason is a new rebel coalition, founded in March, called Jaish al-Fatah, whose name means the Army of Conquest. Jaish al-Fatah, which includes Jabhat al-Nusra and several other rebel factions, has proven remarkably effective in combat with Assad's forces. Moreover, per Robinson, "the Assad regime has had trouble, recently, recruiting new members to the military — particularly from the Alawite community."

But it's too early to break out the champagne. There's little indication that either Assad or ISIS is going to collapse imminently. Even if one of them does, Nusra stands to gain the most.

July 2015: Syrian refugee totals crosses the 4 million mark

The four years of fighting and shifting battle lines have been hell for Syria. About 250,000 people have been killed, and roughly 11.6 million people have been displaced from their homes — about half of Syria's prewar population. Of those, 4 million have been forced out of the country entirely.

These refugees are largely housed in overcrowded and underfunded camps in neighboring countries such as Turkey and Lebanon. With little hope of returning home, many of these families are seeking new lives in Europe. The numbers of Syrians heading to Europe have swelled in the past year and a half. You can see this in the surge in asylum applications, shown in the chart below, which helps to show the crisis's growing urgency not just for Syrians but for the world:

The journey is expensive, uncertain, and often fatal, as in the tragic case of Aylan Kurdi, the Syrian boy whose body washed up on a Turkish beach. That these Syrian families would risk so much speaks to the horrors they're fleeing, and to their hopes, however faint, of finding a future for their children.

September 2015: Russia's military formally intervenes in Syria

In mid-September, a few dozen Russian military jets showed up at an aging military base the country uses in Syria, along with a couple hundred troops to guard them. On September 30, Russia officially launched airstrikes in Syria — its first overt combat operation in the war. The bombings were officially sold as an anti-ISIS operation.

In fact, as the above map shows, the strikes did not target ISIS but rather anti-Assad rebels — many of whom also fight ISIS. These strikes are really just an escalation of Russia's long-running strategy of propping up Assad, now by bombing his enemies within Syria.

Russia's offensive isn't yet a game changer in military terms: These airstrikes, on their own, won't be able to change the fundamental dynamics of the war. The offensive might even help some rebel groups' recruiting by inciting outrage among Syria's Sunni majority. The Russian military is notoriously indifferent to civilian casualties.

The escalation, then, is best understood as an act of fear rather than canny strategy. As Amanda Taub explains, Syria has come to represent all of Putin's greatest fears about democratic revolution and the expansion of Western influence. He's determined to try to save Assad, even if it the effort will cost him.

But the escalation isn't totally irrational. Russia's basic strategy is diplomatic, not military: It wants to reverse Assad's international isolation, as well as its own, by positioning Syria and Russia as leading a new coalition against terrorism. That's why, in an address to the UN General Assembly on September 28, Putin called for an "anti-Hitler coalition" uniting Russia, the West, and Assad against ISIS and other unspecified terrorist groups (which, in Putin's mind, likely include the Syrian rebels).

The Obama administration's response to Russia's overtures isn't exactly clear. But Russia's bombing campaign in Syria is definitely a significant new development — particularly for the Syrian civilians trapped in the crossfire.