One nation, two wars, a quarter-million lives lost, millions forced from their homes, and still no end in sight for the pitiless, seemingly bottomless crisis in Syria.

The U.S., Russia and a collection of Middle Eastern and European countries are gathering with Syrians Friday in search of a solution. The urgency of making peace and rooting the Islamic State from its Syrian base is heightened by evidence of the extremist group's spreading influence — the shock of attacks in Paris and San Bernardino, California, extremists boasting of downing a Russian jetliner, and bombings dotting the Middle East.

Yet how to untangle the nearly 5-year-old Syrian mess is a puzzle.

A rundown of who's fighting whom, and why, in Syria:

___

TWO WARS

There are actually two wars going on in Syria. First came the rebellion against Syrian President Bashar Assad.

Assad succeeded his father as authoritarian leader of Syria in 2000. He responded to peaceful Arab Spring protesters in 2011 with a crackdown so brutal that it sparked an armed revolt that still rages.

Then came an invasion by the Islamic State, which Assad considers one of many "terrorists" arrayed against him.

Formerly known as al-Qaida in Iraq, the Islamic State is infamous for videotaped beheadings, mass murder and systematic rape of captive women — and now, inspiring or possibly directing attacks in the West. Syria's chaotic war opened the way for the Islamic State to expand across the border in 2013. IS has seized about a third of Syria, and has placed its capital in the northern Syrian city of Raqqa.

A U.S.-led coalition is fighting the Islamic State from the air, while the CIA is secretly arming and assisting rebels fighting Assad.

___

WHO'S WHO IN SYRIA'S CIVIL WAR?

With Assad:

— The Syrian army and pro-government militias.

— Russia. Longtime allies of Assad, the Russians set up a base in Syria this fall that launches airstrikes and added cruise missiles fired from the sea. Russia says its campaign is meant to weaken Islamic State and other fighters it considers "terrorists"; Western officials say most of the strikes instead have targeted Syrian rebels fighting Assad. Russia has also been pivotal in designing a peace plan for Syria that involves a transitional period under which Assad can stay in office.

— Iran. It bolstered Assad by sending weapons, military advisers, millions of dollars' worth of aid, and Revolutionary Guard troops, as well as rallying Shiite fighters from Lebanon's Hezbollah militia and elsewhere in the Middle East.

Against Assad:

— The Free Syrian Army. A loose coalition of dozens of moderate-leaning armed factions in Syria. They get backing from the West, including the U.S.

— Al-Qaida and its allies. Al-Qaida has severed ties with the Islamic State group. Its Syrian branch is the powerful Nusra Front, and allies in the fight against Assad include the ultraconservative Ahrar al-Sham and Islamic fighters from Russia's restive Chechnya region. Nusra is also fighting the Islamic State.

— United States. While pushing for Assad's ouster, the U.S. hasn't openly taken up arms against him, only the Islamic State. President Barack Obama threatened air strikes against Syria after hundreds of civilians were killed with sarin gas in 2013. Syria, without acknowledging using nerve gas, agreed to relinquish its chemical weapons stockpile in an agreement with the U.S. and other nations.

Covertly, the CIA has been running an operation providing weapons, money and training to thousands of Syrian rebels, including missiles to destroy Russian-made armored vehicles. Meanwhile, Russia's bombs and missiles have targeted these CIA-backed groups.

— Turkey. An early supporter of the rebels, especially Turkmen fighters, who are from a Syrian minority of Turkish descent and mostly live near the border with Turkey.

— Saudi Arabia and Persian Gulf allies. They finance militants, including Sunni extremists, in hopes of toppling Assad. Some are also part of the U.S.-led military coalition.

___

WHO'S WHO IN THE ISLAMIC STATE FIGHT?

The Islamic State:

— The CIA estimates that IS has 20,000 to 30,000 fighters spread between Syria and Iraq. Its skillful use of social media lures young recruits from around the world to the territory where the group says it's building its "caliphate," or state ruled by a supreme religious leader. It's also encouraged followers abroad to attack where they are, in the West or Middle East. IS makes millions from selling oil, ransoming hostages and collecting taxes in the territory it controls.

Against the Islamic State:

— Kurds. An ethnic group without its own nation. The Kurds who live in northern Syria and bordering countries make up some of the fiercest opponents of IS. Their main forces are the People's Protection Units, known by the acronym YPG.

— Syrian Democratic Forces. A U.S.-backed alliance drawing from a mix of religious and ethnic groups, including many Kurds.

— United States and allies. An American-led coalition has launched thousands of airstrikes against IS targets, the vast majority conducted by the U.S. military. Other coalition members who have taken part since the air campaign began in September 2014 are Australia, Bahrain, Britain, Canada, France, Jordan, Saudi Arabia, Turkey and the United Arab Emirates. After the Paris attacks, Germany joined in a noncombat support role.

The only U.S. ground forces in Syria are a contingent of roughly 50 special operations troops who recently began working with local Syrian fighters trying to break the Islamic State's grip on Raqqa, its self-declared capital. A U.S. project intended to train and equip moderate Syrian opposition forces was abandoned as ineffective.

— Saudi Arabia and allies. They've been carrying out air strikes against the Islamic State. This week Saudi Arabia announced the formation of a new 34-nation "Islamic military alliance" to fight IS and work together against terrorism.

— Syria. Assad's government has done little to counter the rise of IS, instead focusing on its fight against rebel forces. Officially, Assad says all Syrians should join together to battle the Islamic State group as a common enemy.

Russia. Says it's targeting IS fighters, even though many of its attacks go elsewhere.

___

WHAT HAPPENS TO ASSAD?

Must Assad go? The U.S. has been saying so for years, while Russia insists that's up to the Syrian people.

Since the Syrian war began, President Barack Obama and his secretaries of state and defense have declared more than a dozen times that Assad must step down. The U.S. argues that Assad — accused of human rights abuses, such as dropping barrels packed with bombs onto civilians — will never be able to steer Syria toward peace.

But going into the latest Syria talks, Secretary of State John Kerry softened the U.S. position that "Assad must go," saying "the United States and its partners are not seeking so-called 'regime change.'"

The Russians have long stood up for Assad, a long-time ally who assures that Syria remains Russia's foothold in the Middle East. Russia has argued that Assad and his army are the only force positioned to defeat the Islamic State.

But lately Russia has emphasized finding a "political solution" to the Syrian crisis, which might mean giving up on Assad, and says the decision should be the Syrian people's to make.

___

SHIITE vs. SUNNI

Like so much in the Middle East, the Syrian war is shaped by centuries-old conflicts between Shiite Muslims and Sunni Muslims.

Who's who?

Shiite:

— Assad is Shiite (the Alawite branch), a minority in Syria.

— Iran is overwhelmingly Shiite and supports Assad.

Sunni:

— The majority of Syria's population, including most of those fighting to overthrow Assad.

— The Islamic State group and al-Qaida.

— Saudi Arabia, which supports the rebels, is a Sunni kingdom.

— Turkey is predominantly Sunni.

— Kurds encompass many religions, including some Shiite Muslims, but most Syrian Kurds are Sunni.

The Syrian civil conflict has become a proxy war in the power struggle between Shiite giant Iran, and Sunni power Saudi Arabia.

___

TURKEY vs. RUSSIA

The conflict between Turkey and Russia goes deeper than being on opposite sides of the Assad question.

Turkey wants to protect the Turkmen population battling Assad. Russian warplanes have targeted those Turkmen fighters.

Turkey shot down a Russian warplane recently, saying the jet was flying in Turkish airspace, which has ratcheted up tensions between the two nations.

___

TURKEY vs. KURDS

Turkey publicly opposes the Islamic State. This year it began allowing the U.S. to fly its warplanes as well as drones out of Incirlik Air Base to strike IS. But the U.S. is pressing for Turkey to do more — especially to block the flow of people and money crossing its border to bolster the IS militants.

But Turkey's role in the anti-Islamic State campaign is muted by its own war with some of the fiercest fighters in the battle against IS: the Kurds.

Turkey has been battling Kurdish insurgents — known as the PKK — and their struggle for autonomy for decades.

Turkey is loath to let up on the Kurds, even to defeat IS.

___

Associated Press writers Zeina Karam in Beirut and Robert Burns, Ken Dilanian and News Researcher Monika Mathur in Washington contributed to this report.