he surprise announcement by Russia on Monday came as suddenly as the airstrike campaign started last September.
"The task that was assigned to the Ministry of Defense and the armed forces as a whole has achieved its goal," Russian President Vladimir Putin said.
But Russia's stated goal -- fighting terrorists like ISIS in Syria -- didn't match the reality on the ground, critics say. They point to the bombings of civilian areas as reason to believe Russia is actually helping its ally, Syrian President Bashar al-Assad, eliminate his opposition.
Russia has both economic and ideological reasons to support the Syrian regime, even as many other countries blame Assad for the deaths of thousands of dissidents.
Analysts estimate Syria has spent billions of dollars on Russian-made defense equipment. And Russia doesn't believe revolutions nor regime change bring stability and democracy. It often points to the Arab Spring and the U.S.-led war in Iraq as evidence.
So it's no surprise Russia launched hundreds of airstrikes in support of Assad over the past few months.
In a phone call between Putin and Assad on Monday, "the two leaders noted that the operations conducted by Russia's Aerospace Forces have brought about a real turnabout in the fight against the terrorists in Syria, throwing their infrastructure into disarray and causing them substantial damage," the Kremlin said.
"In this context, Mr. Putin said that Russia's Armed Forces have fulfilled their main mission in Syria."
As for the Syrian president, Assad "noted the professionalism, courage and heroism of the Russian service personnel who took part in the military operations, and expressed his profound gratitude to Russia for providing such substantial help in fighting terrorism and providing humanitarian assistance to the civilian population," the Kremlin said.
On Tuesday, Russian forces at Hmeymim air base in Syria began preparing aircraft for the flight back to Russia, the state-run Tass news agency reported.
But Russian forces won't leave Syria entirely. An aviation support center in Syria will be maintained to monitor compliance with the country's ongoing cessation of hostilities, the Kremlin said.
The timing of Russia's withdraw is significant for two reasons.
First, Putin's announcement came the same day Syrian peace talks resumed in Geneva, Switzerland. Members of both the Syrian regime and opposition are meeting indirectly through a mediator to try to forge a path to peace.
On the agenda: How to govern Syria, a new constitution and presidential elections.
Some see Russia's withdrawal as evidence that Putin is sending a message Syria must reach a political solution, CNN's Matthew Chance reported from Moscow.
"You can't ignore the timing of this and the symbolism," he said.
German Foreign Minister Frank-Walter Steinmeier also said Assad would face new pressure.
"If a Russian troop withdrawal materializes, it would put President Assad under pressure to finally seriously negotiate a peaceful political transition in Geneva that would ensure the continuation of a Syrian state," he said.
The second reason Russia's timing is significant: This week marks the five-year anniversary of the Syrian civil war.
In those five years, more than 270,000 people have been killed, half the country has uprooted and over a million migrants have made the dangerous voyage to Europe -- leading to an international humanitarian crisis.
The war has taken an especially brutal toll on children.
At least 900 children were killed last year -- "150 of them while they were sitting in their own schools," said Staffan de Mistura, U.N. special envoy to Syria.
And with much of the country reduced to rubble, about half of all Syrian children -- 2.8 million -- don't have access to education, according to UNICEF, the United Nations' children's agency.
The agency said children as young as 3 years old are now working or begging to help sustain their families.