The conflict in Syria is often referred to as a civil war, meaning a conflict between citizens of the same country.
It certainly started as an uprising of Syrian citizens.
They took to the streets in 2011 to demand democracy and an end to corruption, and their opposition to the government of President Bashar al-Assad quickly spiralled into a war.
But if you look at today's headlines, it's clear that what is happening today in Syria is far more complex.
Some of most powerful countries in the world are involved. As are an alphabet soup of armed opposition groups, the Kurds and, of course, so-called Islamic State.
But what's everyone doing in Syria?
It's complicated - but the best way to start is by looking at the war as a conflict between those who, in broad terms, support and oppose Mr Assad and his government.
On the Syrian government's side, we have:
- Russia (carries out air strikes and provides political support at the UN)
- Iran (provides arms, credit, military advisers and reportedly combat troops)
- Hezbollah (The Lebanese Shia movement has sent thousands of fighters)
- Shia Muslim militias (recruited by Iran from Iraq, Afghanistan and Yemen)
And, on the side of the rebels* we have:
- Turkey (provides arms, military and political support)
- Gulf Arab states (provide money and weapons)
- The US (provides arms, training and military assistance to "moderate" groups)
- Jordan (provides logistical support and training)
*The term "rebels" is used to describe a huge and diverse array of fighters, some of whom co-operate with jihadists like those from Hayat Tahrir al-Sham, an al-Qaeda-linked alliance. Different foreign states often back different rebel factions.
At this point, it's also worth noting that although the US supports some rebel groups, it had not intentionally attacked the Syrian government directly until Friday, when it fired missiles at an airbase used in an alleged chemical attack.
Wait, what? I've definitely heard about US strikes in Syria before.
Yes, you have. But those were targeting IS militants as part of an international campaign that began in September 2014.
Lots of other countries are taking part, including the UK, France, Jordan and several Gulf States.
Basically, IS exploited the war in Syria to carve out a large part of the country for the "caliphate" whose creation it proclaimed three years ago.
This is where the battle-lines get blurry. The Syrian government and Russia are also bombing IS, but separately to the US-led coalition.
So think about it like this: Syrian rebels (and their backers) are fighting the Syrian government (and their backers), and both sides are fighting IS. Although IS wants to overthrow the government, it also violently opposes the rebels.
There's just one more piece of the (simplified) puzzle.
Introducing the Kurds
"The enemy of my enemy is my friend," is a saying that doesn't apply in Syria.
Kurdish people living in Syria's north declared the creation of an autonomous government in areas under their control in early 2014.
This added a fourth dimension to the widening conflict.
The Kurds - who say they support neither the government nor the opposition - have been battling IS along the Turkish border.
They have benefitted from considerable military support from the US, which sees them as one of most effective anti-IS forces on the ground.
Turkey is also fighting IS. But in this case, the enemy of their enemy is also their... enemy.
Basically, Turkey sees the Kurdish Popular Protection Units (YPG) militia in Syria as an extension of the outlawed Kurdistan Workers Party (PKK), which has been fighting the Turkish military for decades.
To prevent Syrian Kurds establishing a contiguous autonomous region along its border, Turkey has carried out air strikes on the YPG and backed a Syrian rebel offensive that drove IS out of the last bit of territory not controlled by the YPG.
OK. But, hang on, you never explained why all these foreign powers are involved in the first place.
Russia has military interests in Syria, including its only Mediterranean naval base and an airbase in Latakia province. It intervened at the request of the Syrian government and said its motivation was to fight terrorism.
Analysts say it also wants to bolster its influence in the Middle East, and globally, by being the key player in Syria.
Iran sees Mr Assad, a member of the heterodox Shia Alawite sect, as its closest Arab ally. Syria is also the main transit point for Iranian weapons shipments to Hezbollah in Lebanon.
Regional Sunni powers like Saudi Arabia oppose Iran's influence in Syria, whose population is predominantly Sunni.
The US began backing rebel groups because it said Mr Assad was responsible for widespread atrocities.
But in the last two years it been more focused on the battle against IS, and President Donald Trump suggested only days before Friday's missile strike that getting Mr Assad to leave power was no longer a priority for him.
Tuesday's suspected chemical attack appears to have changed everything.
Read more to understand Syria's war