Foreign intervention in Syria's seemingly intractable conflict is nothing new. Opponents of President Bashar al-Assad — notably the US, Turkey, Saudi Arabia and Qatar — have been supporting rebel groups since an armed uprising erupted in 2011 in response to security forces' brutal treatment of Arab Spring-inspired protest. Regime allies Russia, Iran and Lebanese Shia militant group Hezbollah have shored up government forces via various routes.
Neighboring Israel has occasionally shelled or launched airstrikes inside Syria for the past three years, usually targeting weapons thought to have been destined for longtime foe Hezbollah.
But this year, the fifth in what has become the worst humanitarian crisis of our time, saw an unprecedented level of international involvement.
The motivation, at least notionally, was in many cases the threat of Islamic State militants. The US continued the airstrikes on the group, also known as ISIS or IS, it began in September 2014 along with five Arab countries, after cobbling together an international anti-IS coalition in Paris. American aircraft, which carried out the vast majority of attacks from the start, took an even greater share, according to some reports, allowing the Arab coalition countries to escalate strikes on Houthi rebels in Yemen -- strikes that have killed and injured large numbers of civilians. Canada joined its southern neighbor in April, when two of its aircraft hit targets in Syria for the first time.
Then, after the devastating IS-claimed attacks in Paris, French aircraft launched a number of airstrikes in and around the extremist group's de-facto capital of Raqqa. The UK House of Commons approved bombing in Syria on December 2, in an expansion of combat sorties against IS in Iraq that Britain had been conducting since the previous year.
But it was a significant military intervention by Russia that had by far the greatest impact. Moscow has been providing arms and military advisors to Assad since the beginning of the war. But after pro-government forces suffered a string of defeats, most notably losing Idlib province to rebels in March, the Syrian president's position looked shakier than at any point in the war thus far. Concerned, Russia and Iran held high-level talks about providing coordinated support to their mutual ally.
Russia began in August by deploying jets, tanks and personnel to an airbase near Latakia, as well as stationing some its Black Sea fleet off the Syrian shore, prompting more and more rumors of Russian troop involvement on the government side. It also agreed to establish a "joint information center" in Baghdad with Iran, Iraq and Syria, with the claimed goal of combatting IS.
On September 30, Russian officials gave the US an hour's notice and launched strikes in the northwest of the country, apparently in response to a formal request made by Damascus.
Contrary to initial claims, most of the initial attacks targeted non-IS rebel groups, ranging from the hardline al-Qaeda affiliate Jabhat al-Nusra, to Islamist factions and US-backed members of the Free Syrian Army. Russia flatly claimed it was hitting only IS, despite its own data and images contradicting this, before eventually saying it was targeting "terrorists", a blanket term used by Assad's government to refer to all armed opposition groups.
The UK, US, France, Germany, Qatar, Saudi Arabia and Turkey all expressed "deep concern" at Russia's military activities in Syria in a joint statement, and called on Moscow to "immediately cease" attacks opposition opposition groups and civilians. "These military actions constitute a further escalation and will only fuel more extremism and radicalization," the statement added.
On the sidelines, Israel set up a communications protocol for coordinating activiities over Syria with Russia and has continued its own operations, including allegedly killing senior Hezbollah member Samir Kantar along with several others in an airstrike on an apartment building just outside Damascus in December.
Unsurprisingly, Assad welcomed increased Russian involvement, telling Hong Kong's Phoenix television that since the bombing campaign begun, things had "improved in a very good way". He went on to claim that his troops were now "making advancement in nearly every front". As yet, the increased air support appears not to have given his forces any decisive battlefield advantage.
Russia also launched cruise missiles on Syria. The first 26 were fired from the Caspian in October through Iraqi and Iranian airspace. Four crashed in Iran, CNN reported, claims both Moscow and Tehran denied.
It has since launched several more from the Caspian, closing Iraqi airspace and forcing airports in the semi-autonomous Kurdish north to shut down for 48 hours. In December, Russian officials said that missiles had also been fired into Syrian territory from submarines in the Mediterranean.
Russia says its air force is conducting "pinpoint" strikes, but footage and still images show it also used unguided bombs dropped from high altitude on numerous occasions. And while Russia has hit IS on occasion, activist group Raqqa is Being Slaughtered Silently said even these strikeshave had little effect and sometimes seemed to be directly targeting civilians.
In this map from IHS Conflict Monitor, the Assad regime is shown losing 18 percent of its territory between January and August 2015, just before Russia decided to intervene in its favor in Syria.
And Russia's air operations in Syria do seem to have taken a significant toll on the civilian population. The UK-based Syrian Observatory for Human Rights (SOHR) said that in two months of bombing, Russian strikes killed 485 civilians as well as 419 IS members and 598 other opposition fighters. The Syrian Network for Human Rights (SNHR), a separate monitoring group, said in mid-December that the toll was 570 civilians, including 152 children and 60 women. On December 20 alone, a number of Russian strikes, including on a courthouse, were reported to have killed dozens in Idlib.
By comparison, SOHR said in October that in more than a year, the US-led coalition strikes had killed 226 non-combatants, as well as 3,276 IS members. SNHR put the civilian toll at 225. The US has only admitted to a handful of civilian deaths, however.
A recent Human Rights Watch report accused Russian and Syrian forces of making extensive use of cluster munitions in Syria, inherently indiscriminate and internationally banned weapons that HRW said have been responsible for a number of civilian deaths.
Russia's Syrian campaign also led to the first international incident around the Syrian civil war. On November 24, a Turkish F-16 shot down a Russian Sukhoi Su-24 bomber that violated Turkish airspace for a few seconds — the first such engagement between Russia and a NATO member state in decades. Political posturing and retaliatory sanctions followed.
While Russia backed Syria's regime in the air, Iran boosted its presence on the ground. The Syrian army, devastated by heavy losses and desertions, is short on manpower, something Assad himself admitted in a July speech. But his allies came to his aid. A number of reports detailed arrivals of hundreds of Iranian Revolutionary Guards Corps troops and an expansion in Tehran's role.
Foreign fighters have been increasingly prominent at the highest level of pro-government forces. A Syrian army defector told Middle East Eye that most of the fighters in his southern Syria-based division were Iranian Revolutionary Guards Corps or Hezbollah and that Iranians controlled the division, barring Syrian regulars from even accessing operations rooms.
In June, it was reported that Iranian commanders in Syria had executed three government troops south of Idlib for "betraying the homeland," after the trio withdrew from checkpoints. Syrian officers present weren't able to prevent the killing because Iranian officers ran operations in the area, the report added.
But the increased Iranian involvement has led to Iranian casualties, with dozens of fighters, including a number of officers, killed or wounded in the past few months. The commander of the elite Quds Force, Qassem Suleimani, was even rumored to have been injured in late November outside Aleppo. Some reports said that Iran is now pulling out some troops as a result.
Hezbollah is also understood to have increased its presence in recent months, as have Iraqi Shia militias, many of which are backed by Iran.
2015 then was the year that much of the world got involved in Syria. In Canada, however, newly elected Prime Minister-Justin Trudeau announced that he planned to withdraw Canadian jets from the anti-IS coalition. Amongst escalation from all sides, his was an isolated decision indeed.