According to textbooks on political violence, a terrorist attack requires the following key components: a target, an actor or actors, and motivations.
The Kremlin's decision to launch air strikes in Syria could raise the risk of terrorist attacks on Russian soil, security analysts said, as the move increases the motivation of Russia-related supporters of the Islamic State to retaliate against Moscow.
As its forces waged security sweeps and real combat operations in the Northern Caucasus during the past two decades, Russia has been targeted by terrorists many times, but in the past 18 months the security forces have managed to contain them, at least from committing massive acts of deadly violence.
"Islamists, jihadis and extreme Muslims are, to put it mildly, not pleased with Russia's interference in Syria and they will be prepared in some way to answer what is happening at the moment," Sergei Goncharov, a terrorism expert who heads an organization for veterans of Alpha, a Russian special forces unit, told The Moscow Times.
Russia's Defense Ministry said Monday in an online statement that its planes had flown 25 sorties in Syria in the past 24 hours and had hit nine Islamic State targets.
The Britain-based Syrian Observatory for Human Rights said 14 fighters, most of whom belonged to the Islamic State, had been killed since Russia launched its air offensive on Wednesday, Reuters reported Saturday.
The threat of retaliation is not a new one: Even before Russia's massive buildup of arms in Syria over the last month, Islamic State counted Russia among its enemies.
In September last year, after claiming to have seized a Russian-made MiG fighter jet at a military base in the Syrian province of Raqqa, IS militants for the first time openly declared war on Russian President Vladimir Putin.
"This message is for you, Vladimir Putin! These are the aircraft you sent to [Syrian President] Bashar [Assad], and we're going to send them to you. Remember that!" a militant could be heard saying in a video uploaded to YouTube at the time.
But the decision to launch air strikes in Syria could be a trigger to translate words into actions, experts said.
Around 2,400 Russians — mainly natives of Russia's southern predominantly Muslim-populated regions of Chechnya and Dagestan — are fighting together with Islamic State militants in Syria, Sergei Smirnov, first deputy director of Russia's Federal Security Service (FSB), said last month.
Nearly all major terrorist attacks in Moscow, from the Nord Ost theater hostage crisis in 2002 to the twin metro bombings in 2010, were perpetrated by natives of these regions.
While those people are fighting in the Middle East, Russia is safer, said Simon Saradzhyan, a research fellow at the Belfer Center for Science and International Affairs, a Harvard University policy think tank.
"As long as they are there, the threat [of terrorism in] Russia has actually diminished," he said. "The more [militants] Russia annihilates on the ground [in Syria], the fewer will come back."
But according to Alexei Malashenko, an analyst at the Carnegie Moscow Center think tank, the real number of fighters from Russia and Central Asia who have joined the Islamic State is closer to 7,000, and hundreds of them have already returned to Russian soil.
The Russian government stepped up security at home after launching the air strikes in Syria.
Presidential spokesman Dmitry Peskov said Thursday that the county's National Anti-Terrorism Committee, special security forces and FSB were on alert.
"They are constantly taking measures to prevent [terrorist attacks]," Peskov was cited as saying by state-run news agency RIA Novosti.
Terrorists strive to hit symbolic targets and generate as strong a media storm as possible. In Russia, they have taken hundreds of hostages, including children, sent teams of female suicide bombers on coordinated missions, and targeted Moscow. The concentration of the media — including international outlets — is the highest in the capital, which is the most targeted Russian city outside the North Caucasus.
"The likeliness of a terrorist attack is very high, and it will probably take place in a large city like Moscow, to make a political point. But we cannot know when," said Malashenko.
Previous attacks were primarily aimed at forcing Russia's hand in the North Caucasus, not abroad, and proved unsuccessful in pressuring the Russian government to change its policies with regard to Chechen separatists and the radical Muslim underground in the North Caucasus.
But terrorist organizations have often hit targets abroad in order to force foreign policy change, such as the 2004 bombing of commuter trains in Madrid by an al-Qaida-inspired terrorist group when Spanish troops were supporting the U.S. military campaign in Iraq.
Russia has a mixed record on negotiating with terrorists — the hostage taking in the Stavropol region city of Budyonnovsk in 1995 was widely seen as a turning point in the first Chechen War by forcing the Kremlin to return to the negotiating table with Chechen rebels.
But since Putin took the helm in 2000, there has been an unbending policy of non-negotiation with terrorists, said Saradzhyan.
Putin stood firm in refusing to give in to Chechen separatists even after the 2004 Beslan school hostage taking that shocked the world, in which over 350 people were killed, more than half of them children.
"Even if the Islamic State manages to stage successful attacks on Russian soil, it will not change Russian policy [in Syria]," Saradzhyan said.
Most analysts said the air strikes were unlikely to translate into immediate terrorist attacks. However, the danger will grow if Russia is drawn into a long conflict.
"The question is whether the Kremlin gets seduced into the pattern, so often visible when countries embark on interventions abroad, of thinking one more push, one more expansion of forces will make a difference," Mark Galeotti, an expert in Russian security services and a professor at New York University, told The Moscow Times in written comments.
And while Moscow's air strikes could eliminate some of its enemies, it could also sprout new ones.
Western leaders have repeatedly expressed concern that Russia is targeting not only the Islamic State, but also U.S.-backed groups like the Free Syrian Army.
Broadening its line of fire to include moderate groups will not win Russia new friends and could sow the seeds for a new wave of violence, Saradzhyan said.
"Inevitably, these groups will develop grievances vis a vis Russia and will try to avenge," he said.
More immediately threatened could be Russians based in Syria: both military specialists and "ordinary Russians who reside in Damascus and other cities," said Yury Barmin, a Middle East expert.
"I also think that Russia's involvement in Syria may be risky for Orthodox Christians who live in Syria's coastal region," he added.
The head of the Russian Orthodox Church, Patriarch Kirill, publicly backed the air strikes last week, and Vsevolod Chaplin, a spokesman for the Moscow Patriarchate, called the fight against terrorism in Syria "a holy war."
"The endorsement of the Syria operation by the Russian Orthodox Church gives extra impetus to the extremists," Theodore Karasik, a senior adviser for Gulf State Analytics, a consultancy, told The Moscow Times.