ere are many separate wars happening within Syria and Russia, Iran, Turkey, and the US all have a stake. Russia is now the dominant international force in Syria and they're blaming the US for being behind the current crisis. For encouraging quote, separatist sentiment among Syria's Kurds. And the trigger for Turkey's operation now could have been talk of the US creating a border force in Syria with Kurdish militia.
Turkey has been saying for months that it is concerned about what they see as a terrorist corridor in Northern Syria. The map of control in the war torn country is constantly being redrawn and as a patch work of the Syrian regime backed by Iran and Russia, Islamic state, Syrian rebels, and Kurdish forces.
There's also a Turkish buffer zone in the north. After Afrin, Turkey says Manbij to the east is next, and that's where it's likely to get more complicated.
This made Turkey furious and relations haven't been the same since. While the US doesn't have forces in the areas around Afrin, they're doing man bait to the east where there's around 200,000 American soldiers and their allies raising the possibility of conflict between groups allied with Turkey and the US.
Turkey's government says it only wants to battle the terrorists, but with so many competing interests, avoiding a wider fallout will be difficult.