The Kurds must be banging their heads against the wall for their stupidity in attacking the Syrian Army in Hasakah. They fueled Turkish fears about a Kurdish state - and doing so gave the Turkish army an excuse to invade. At the same time they antagonised the Syrian Army - that otherwise might have done something to help them. And by attacking Syria's legal government they reduced the threshold for the Turkish army to violate Syrian sovereignty. Either they were really stupid or their adversaries seduced them in some very clever Machiavellian scheme to do some very stupid things. As the Kurds have said that they don't do anything without the consent of the US, that raises awkward questions about the role of the US here.
They might not have known the details, but the Kurds could have known they made a mistake. The time just isn't there for a Kurdish state with Turkey and Iran rabidly opposed and in a position to fight it. Not to mention that neither Iraq nor Syria is resigned to let its Kurds go and that in neither country the borders from such a Kurdish state are clear. So the smart thing for the Kurds is to get a good position for when negotiations will take place. And in the case of Syria keeping their liberal autonomy without the repression of Assad is probably the maximum that can be achieved for now. And even that only once the war is coming to an end.
Another issue is that there are much too much Arabs living in the north of Syria - specially in the middle where they constitute a wide majority of the population. Assuming that a Kurdish state will cover that whole territory is just not realistic. Not to mention that it alarms both Turkey and the local Arab population. So the smart thing would have been to keep quiet and claim that they were only building temporary structures and that only after the war there would be negotiations where a definitive solution would be decided - taking into account the wishes of the local Arab population. Even for Kurds with more radical ideals it would have been wise to adopt that position for tactical reasons.
After having conquered Manbij the Kurds were in a position to make life hard for ISIS. If they conquered Al Bab - what they declared to be their next goal - they would be close to the government held Kuweires airbase east of Aleppo. Connecting with them would cut the ISIS territory in two. That would cut their supply lines to Turkey - generally believed to be a supporter of ISIS. It looks like that prospect was too much both for Turkey and the US. So this is in essence about saving ISIS. And while the Turks attacked Jarabulus the Americans put heavy pressure on the Kurds to withdraw from Manbij. Reports about whether they are complying are contradicting.
That impression is strengthened with the first reports from the ground that there were almost no casualties: one rebel and zero Turkish soldiers died. Locals reported from Jarablus that ISIS fighters went to Turkey by car and came back with a different uniform.
The situation in South Syria - where ISIS maintains territories that can only be supplies via territories held by other rebel groups - provides little hope that the rebels that Turkey is sending in will do anything against ISIS.
Turkish public opinion generally is opposed to foreign adventures like this one. They fear getting stuck in swamp at the cost of the lives of many Turkish soldiers. ISIS could easily have played on those fears by making the Turkish entry deadly. It didn't. On the other hand we saw on the eve of the attack a bombing of a Kurdish wedding in south Turkey. This may well have been a false flag operation by the Turkish army to prepare the popular mood for the operation. Both the target - Kurds - and the conflicting information about this attack - Turkey later withdrew its initial claim that the attack had been made by a teenager - suggest so.
Initial reports from Turkey claim that 15000 troops are ready to participate and that the aim is to secure the border between Jabavulus and Marea (some 70 km). "We want to secure the border before moving south", according to a rebel. The first reports of clashes with Kurds from the SDF are there.
One puzzling question is the role of the Syrian government and Russia. Both reacted very slow and reticent. They certainly can't complain that the action surprised them as the preparations were clear. The head of the Turkish secret service Fidan has been in Damascus frequently recently and very likely discussed the issue. There may be a secret clause in old treaty that gives Turkey the right to invade to fight PKK. Moscow too knew about the attack before. Very likely both fell for the "Kurdish state" red herring. What Turkey is now implementing is something it asked for already three years ago when the Kurds were hardly visible. So although they may now have more reasons to point to Kurdish expansionism, I can't see that as the main reason for Turkey's invasion. Neither do I expect that Turkey will limit its actions to what serves that purpose.
In my opinion the Kurdish rise was mainly a threat to Turkey because it gave the PKK hope. However, for Syria it never was a serious threat. Yet the hysteria is widespread. There are now even reports about a new Arab anti-YPG group in Afrin.
Finally there is the US position. Some people have explained Biden's appeal to the Kurds to withdraw East of the Euphrates as a sign that the rift between Ankara and Washington is not real and may just have been theatre to build bridges towards Moscow. Some see even the coup as a theatre co-production between them. However, I tend to the view that it was real. I read some where that Biden was met at the airport by a low ranking official. My impression is that the Turks told the US that they would invade Syria and that they told that to the US as a matter of fact, that Biden went to Ankara to save what could be saved and that the Turks told him that the best the Kurds could do is to get out of the way and east of the Euphrates. Erdogan may even be blackmailing the Americans - for example by threatening to close Incirlik for them.
In the mean time Obama keeps talking about Raqqa. But conquering Raqqa is useless. The city is a symbol because it was the first big city conquered by ISIS. But conquering symbols is not a strategy. It is not impossible to get the city, but what then. It will be like Palmyra. It was conquered a long time ago by the Syrian army yet it stays vulnerable to attacks from ISIS. In the open desert one would need to conquer a perimeter of at least a hundred kilometer to be a bit safe. Unfortunately it looks like Obama isn't interested in a structural solution. He only wants a symbolic victory and the accompanying positive pr.