A look at the factions battling in Syria’s civil war

— -- In the tense days after U.S. airstrikes on the regime of Syrian President Bashar al Assad, Russia’s ally, the U.S. and Russia have traded accusations about siding with terrorists.

"Russia has really aligned itself with the Assad regime, the Iranians and Hezbollah," Secretary of State Rex Tillerson told reporters in Italy before his Moscow trip. Hezbollah is a Lebanese militia group that the U.S., the Arab League and others deem a terrorist organization and have sanctioned.

Russia has condemned the U.S. for supporting terror groups, as well.

"It’s not difficult to imagine how much the spirits of terrorists have been raised by this action from the United States," Deputy Ambassador to the U.N. Vladimir Safronkov told the Security Council last Friday.

Are there any merits to either accusation? Like with many things in Syria’s six-year civil war, it depends on who you ask.

What started as a civil war and a rebellion against an oppressive regime has become a broader conflict with splintered opposition factions fighting both the government and each other. In addition, several countries in the region and outside actors have armed and supported those separate groups.

These are the main players to know on the battlefields of Syria.

ASSAD AND HIS ALLIES

Once thought to be on the edge of collapse, the Assad government is now in control of a significant portion of the country after beating back rebels and Islamic militants. The Assad government's fortunes have been bolstered by its air force superiority and the assistance of Russian artillery, troops and additional air support.

Longtime allies, the Russians have poured billions of dollars and immense political and military capital into propping up Assad. In exchange, Russia gets access to a Mediterranean port and an important international power base.

Assad belongs to the Alawite sect of Islam, a branch that is related to Shi'ism. He is supported by the region's largest Shiite power, Iran. In fact, Iran has lost more than 1,000 soldiers in Syria, including generals from its Islamic Revolutionary Guard Corps, according to an official estimate from the Iranian government. The casualty count could be even higher. The U.N. estimated at one point that Iran was spending upwards of $6 billion a year supporting Assad.

"To Putin, Syria is strategically useful; to Iran's regime, it's existentially important," said Iyad el-Baghdadi, a human rights activist and fellow at the Norwegian think tank Civitas.

Even more directly involved on the frontlines is Iran’s ally Hezbollah, the Lebanese militant group responsible for the 1983 Beirut barracks bombings that killed 241 American service members.

For years, Syria was the conduit through which Iran sent arms and funds to Hezbollah. When that lifeline was threatened by recent civil war, both sides started pouring support into Assad. Now, Hezbollah’s fighters are involved in major campaigns alongside Russian forces and have sustained at least a thousand fatalities. They are also engaged in social programs and training other Shiite militias.

Together, these forces released a statement in support of Assad after the U.S. airstrikes, vowing to retaliate if the U.S. took further military action. The remarkable statement prompted Tillerson’s comments and led to this ultimatum from America’s top diplomat.

"Russia can be a part of that future and play an important role, or Russia can maintain its alliance with this group, which we believe is not going to serve Russia's interest longer term," he said Tuesday. "But, only Russia can answer that question."

TERRORIST GROUPS

Outside of the alliance supporting Assad, the fault lines and factions get murkier. But two groups are almost unanimously labeled as terrorist organizations, battling nearly every other side: ISIS and al-Qaeda.

The infamous and brutal terror group that rose in the chaotic vacuum of Syria’s civil war, ISIS continues to hold slices of territory and operate out of Raqqa, its de facto capital in the north central region of the country. Though ISIS remains a player on the battlefield, it has been losing ground to the Assad alliance, as well as the Global Coalition to Counter ISIS.

Al-Qaeda’s role in the conflict is more shadowy. ISIS started as an al-Qaeda affiliate, but the two groups split in early 2013. In the wake of the division, al-Nusra Front emerged as al-Qaeda’s offshoot and the U.S. targeted them with airstrikes on a handful of occasions.

As the group tried to blend into the conflict and avoid Western attacks, it claimed to have broken ties from al-Qaeda, changing its name and adding factions. Now, it is known as Tahrir al-Sham, although many analysts believe it is still al-Qaeda’s affiliate on the ground. The U.S. government added the group to its list of terrorist organizations that are sanctioned, but at times moderate opposition forces fight alongside it.

GLOBAL COALITION AGAINST ISIS

The Global Coalition taking on ISIS was founded by the United States and now has 68 member nations and organizations like the EU, NATO and Interpol. The coalition has conducted heavy strikes in Iraq, helping Iraqi security forces liberate cities like Tikrit and Fallujah and is now supporting their campaign to retake Mosul.

But in Syria, there is a greater challenge because the coalition has no government partner. Instead, it is relying on a coalition of local forces some members have helped train and equip to seize ground as it conducts airstrikes. It was the model used in Kobani, a northern Syrian town on the border with Turkey. ISIS now has one last major stronghold -- Raqqa.

Together, the coalition has no official position on Assad, just a focus on defeating ISIS. But individually, many of these countries, including the U.S., have said he should no longer be considered a legitimate ruler for the Syrian people.

U.S.-BACKED FORCES

The strongest of those local forces taking on ISIS and supported by the coalition are the Syrian Democratic Forces (SDF), a diverse coalition of Kurdish, Sunni Arab and Syriac Christian fighters that helped retake Kobani and Hasakah and are now bearing down on Raqqa.

"We’re working to build a diverse coalition that is rooted in the local population to take and hold ISIS’s capital city in Raqqa" a senior State Department official told reporters last week. "The principle is that local people from local areas will control those territories, and we can help with basic stabilization and humanitarian support."

The largest, most dominant group within this coalition is the Kurdish YPG, a Syrian group with connections to Turkey's PKK -- a Kurdish independence group that the government there, as well as the U.S., consider a terrorist organization.

Rebel groups focused on fighting the civil war still exist, as well. The Free Syrian Army was once the heart of the opposition that some American leaders like former CIA director David Petraeus and former Secretary of State Hillary Clinton wanted the U.S. to arm. Now, the group has become a larger, but more decentralized umbrella group of rebel factions. They are still focused on toppling the Assad regime.

Some American officials like Senator John McCain have been advocating for the U.S. to step in again and provide extensive support to the FSA, but others, weary from Iraq and Libya, don’t want the U.S. to get further involved in Syria’s conflict.

OTHER OPPOSITION FORCES

Another key force on the battlefield is Ahrar al-Sham, an umbrella group of hard-line militants that want to replace Assad with an Islamic government. Despite some overlap in ideology, they also fight against ISIS and, at times, Tahrir al-Sham. Along with the FSA and some other Islamic groups, these are the principal forces on the front lines that Assad battles.

Many of these groups have been secretly supported and armed by Saudi Arabia and other Arab countries like Jordan and the Gulf states.

The Assad regime has been quick to label anyone in the opposition against it a "terrorist," accusing those groups' Western and Arab allies of supporting terrorism. Russia considers Ahrar al-Sham to be part of the "moderate opposition" included in its ceasefire negotiations with Iran and Turkey, but the group declined to attend those talks.

TURKEY

Last, but certainly not least in battlefield capability or influence is Turkey, Syria’s neighbor to the north. Turkey remains a fickle, but critical American ally.

Turkey launched a ground campaign in northern Syria last August, sending troops, tanks and planes across the border to push back ISIS. That operation, known as Euphrates Shield, has concluded, liberating key cities like Jarabulus and Manbij. But Turkey remains heavily involved in the conflict, especially through its support for opposition groups.

The U.S. considers Turkey a key ally in the fight against ISIS, but Turkey remains angry about American support for Kurdish groups like the YPG, which it considers a terrorist organization tied to the PKK. The Turks are very wary of rising Kurdish power and fearful of Kurdish aspirations for an independent state on Turkey's southern border.

That tension has led Turkey to thaw relations with Russia, despite its desire to see Assad removed from power. The two countries’ autocratic leaders have been meeting more often in recent months and, along with Iran, their teams have been trying to hammer out a peace deal outside of the U.N.-sanctioned negotiations in Geneva.