Explaining Turkey's reluctant response to Syria

Some might wonder at Turkey's reluctance to engage in Syria even when being given the green light by the world's superpower the mighty USA. Turkey is of a size to easily dominate this smaller and poorer neighbour, so with the world superpower backing them why aren't they taking their opportunities?
The answer is that Turkey knows it is sitting on a powder keg and it cannot afford to antagonise anyone who could light a match.
As Turkey has not managed to successfully integrate its Kurdish regions it knows that a strong push for an independent Kurdish nation is a threat to its territorial integrity. As there are already around thirty million Kurds living in a relatively cohesive area the stage is set, as it has been for a long time. Two big  factors have worked against the creation of a Kurdish state historically: Kurdish disunity and the the lack of a significant external backer. Continuing this state of affairs and preventing the Kurds from gaining a powerful international backer must be a priority of Turkish foreign policy, afterall guarding the home territories must always take precedence over exerting influence abroad. Why Turkey has been so reluctant to engage with the USA in Syria is because it fears that if it allies itself fully with the US then Russia will see it as an enemy and will be tempted to start supporting the Kurds. Russia may not be anywhere near as powerful as the USA, but it is certainly powerful enough to make an independent Kurdistan a realistic possibility if it lends its support to the venture. Turkey consequently has been reluctant to antogonise the Russians either by acting against their ally Al Assad or by joining in with US operations against Islamic State. Until recently that meant that Turkey was not even allowing the US to use its airbases.

So what changed Turkey's mind? Why has it now agreed to work with the US?
The answer it would appear is the looming threat that if Turkey did not stop stonewalling the US then the US would turn for help to the Kurds themselves. As the Kurds have proven themselves in both Iraq and Syria as the only effective fighting force that has broad appeal to western populations (hence the numbers of western volunteers beginning to travel to the Middle East to fight alongside the Kurds) the Turks realise that there is a danger of them becoming a cause celebre in the west.  Western nations have already begun directly arming the Kurds of Iraq. The Turks would also have been aware of how well the Kurds were doing in Syria. Where previously the Kurds had controlled three separate pockets of territory along the Turkish border, during the civil war so far they have managed to take control of the land between two of these pockets so that they now control the vast majority of Syrian territory adjacent to the Turkish border. It is notable that the area that the Turks have supposedly agreed to occupy within Syria is the area which separates the two Kurdish regions, effectively blocking them from joining up and becoming an even stronger territorial entity.

What can we expect of Turkey now?
At this point, Despite having agreed to American demands it appears that the Turks are still trying to avoid Russian ire by telling them that they were forced into co-operation. This is a narrative which tallies with the slow and reluctant actions the Turks have taken on the ground so far (except for their enthusiastic bombing of PKK camps, the Kurdish separatist movement that operates within Turkey). The Turks are making a show of being reluctant to enter Syria for the benefit of the Russians, but they would also hope to be sending a similar message to their Arab neighbours who have resentful memories of Turkish colonialism during the Ottoman period. By their circumspect movements into Syria so far the Turks are trying to say 'We are only in this because we have to be!'