March 15, 2017 marked exactly six years since the beginning of the Syrian conflict.

What began as another pro-reform uprising in the Arab Spring of 2011 quickly deteriorated into something far more grim due to Syria's unique demographics, President Bashar al-Assad's fierce crackdown, and the war in Iraq next door.

Six years later, the Syrian war has left hundreds of thousands of people dead, fuelled the rise of the Islamic State group, and caused the biggest asylum seeker crisis since World War II.

And in the most recent development, a chemical weapons attack on Syrian civilians by Mr Assad's regime has prompted a US missile attack on a Syrian airbase, a move that threatens to sharply escalate tensions.

Here's a look back at pivotal moments and repercussions that have shaped the war in Syria:

Political uprisings

Anti-government protesters holding Syrian flags during a demonstration in the city of Hama. ( AFP: Ugarit News, file )

On March 15, 2011, major protests broke out in the Syrian cities of Damascus and Daraa following weeks of smaller protests throughout the country.

Similar to Tunisia and Egypt before it, Syrian protesters, Islamist and secular alike, demanded freedom and justice from the 40-year-old Assad regime.

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Mr Assad announced conciliatory measures, such as lifting the 48-year-old emergency law, dismissing the government and releasing political prisoners, but at the same time, security forces were shooting protesters dead.

In the days and weeks that followed, Syrian regime forces upped their offensive.

Tanks were deployed to various cities to fiercely crush protests, prompting some in the opposition to respond with gunfire.

Reports continuously surfaced condemning violence on both sides, while the death toll steadily grew.

Many regime forces and senior officers, ordered to kill protesters, eventually defected to the other side, leading to the creation of the Free Syrian Army and other rebel groups and the first militarised takeovers of major towns.

Anti-government sentiment eventually trickled into other major cities like Homs and Aleppo, Syria's largest city.



Civil war breaks out

Syrian soldiers, who defected to join the Free Syrian Army, hold up their rifles as they secure a street in Damascus. ( Reuters: Ahmed Jadallah, file )

Encouraged by offensive successes in smaller but pivotal towns like Daraa, the Syrian military moved to reclaim larger cities like Homs, kicking off the warring domino effect in major cities.

Meanwhile, armed factions continued to grow in strength and number while diversifying in ideology, leading to the formation of dozens of splinter groups.

The fighting reached a tipping point in 2012 when the war engulfed Aleppo, Syria's biggest city, which would not be recaptured by the government for another four years.

Armed opposition groups now included Islamists, Kurds, Al Qaeda, as well as secular fighters.

Over time, disagreements over targets and an increasing number of unclaimed attacks and bombings led to infighting between splintered opposition groups.

Foreign powers also got involved, like the West and Turkey, who trained Syrian opposition forces and supplied them with weapons, while countries like Russia and Iran aided Assad's forces.

In 2013, UN reports surfaced that Mr Assad had killed hundreds of Syrians by means of chemical weapons, which then US president Barack Obama had previously said would be a "red line".

Mr Assad's crossing of that line radically changed US President Donald Trump's attitude to the conflict, shifting from a "stay out of Syria" mindset to one in which he launched strikes.

Islamic State rises

An Islamic State militant on top of a tank in Syria's northern Raqqa province. ( Reuters, file )

Just months after the violence kicked off in 2011, Islamic State of Iraq (formerly Al Qaeda in Iraq) leader Abu Bakr al-Baghdadi began sending forces across the border to establish a Syrian ally.

After recruiting hardline Islamists released from prison and developing a fierce reputation among the array of armed Islamist groups, Baghdadi's proxy group declared themselves as the Al Nusra Front in early 2012.

In 2013, after a series of victories throughout Iraq and Syria, Baghdadi declared a merger between the two groups, leading to the creation of the entity known today as Islamic State.

Al Nusra later rejected the merger, saying it had changed its focus to overthrowing Assad rather than creating a caliphate, causing friction between Al Qaeda and Islamic State militants as well as within its own ranks.

Reports say Baghdadi then travelled to Syria to gather those who pledged allegiance to the greater Islamic State idea.

After months of notoriously brutal yet strategic military takeovers, Islamic State began making headlines as the most powerful Islamist group, most notably in June 2014 when it declared its caliphate months after Al Qaeda disavowed the group.

The group then claimed its de-facto "headquarters" in the Syrian city of Raqqa.

Islamic State has long been both a destabilising and polarising force in Syria's war, making moderates of former Islamist extremists on the international stage, and further convoluting the civil war.



Refugee crisis

Asylum seekers, many of whom are Syrian, break through police lines on the Croatian-Serbian border. ( AFP )

Syria's asylum seeker crisis has seen several "waves" since the unrest broke out in 2011, which often fluctuate directly in response to major violence and warfare.

The UN estimated that by the end of 2016 around 13.5 million Syrians had fled their homes — 8.7 million internally displaced and 4.8 million having crossed the borders into neighbouring countries.

The first wave of asylum seekers began pouring north into Turkey just months after the 2011 clashes broke out.

By the end of the year, tens of thousands of people had also fled to Lebanon, Jordan, Libya and Egypt.

In 2012, after violence escalated in the cities of Homs and Aleppo, this number grew exponentially into the hundreds of thousands, reaching a tipping point in 2013, when the number soared past the 1 million mark as thousands crossed the border daily.

The next major migration wave came in 2014 following the brutal rise of the Islamic State group and the persecution of religious groups, prompting many asylum seekers to try find a permanent home overseas.

By mid-2015, the asylum seeker crisis was felt worldwide as hundreds of thousands of people poured into Europe, bringing the crisis to countries like Germany, Greece, Macedonia and Sweden.

Turkey is still the biggest host of Syrian asylum seekers, with Lebanon, Jordan, Germany, Greece, Macedonia, Hungary, Serbia, Iraq, and Egypt also bearing the brunt of the crisis.

Multiple proxy wars

Who is supporting who in Middle East conflicts? ( ABC News: Steven Viney )

The rise of Islamic State attacks around the world, as well as the growing European asylum seeker crisis, has brought the Syrian civil war to the West's doorstep and increased the ante across the board.

The unrest in Syria has also led various allies and foes of Mr Assad to butt heads over the years, as countries like Saudi Arabia, the US and Turkey trained and backed the opposition, while Iran and Russia backed Mr Assad's regime.

Commencing in 2014, the US-led operation against Islamic State militants was long the only consolidated air operation in Syria.

But it was soon complicated by Russia, which began bombing its own designated "terrorists" — not necessarily those from Islamic State — in 2015.

Turkey, meanwhile, has been using the unrest to settle old scores with the Kurds who have to-date been the most successful at fighting Islamic State militants on the ground.

The US, Russia, Turkey, Iran and Saudi Arabia — the biggest players but not the only ones — all have different geopolitical interests on the line.

Several rounds of UN-brokered Syrian peace talks between oppositional forces and Assad's regime have taken place, but continue to be fraught with problems and marked by delays — and there are questions over how the Trump administration will affect the US' role in the talks.

The fate of Mr Assad remains the single most divisive issue surrounding how to create a roadmap forward and end the conflict.

Mr Assad's use of chemical weapons on his own people, and the swift and decisive response it drew from the US, promise to shift the dynamic again in this already complex war.

