Internationally; this relates to the regional politics, powers and to the overall global balance of powers in a big picture view.
Regionally it is a struggle between two major rising powers, both are looking to take Middle East (ME) under its influence. One is Turkey, presenting itself as a rising regional and international power claiming ME as a strategical depth, in the process opening a market for its mass producing factories, also representing western powers interest to secure power resources, while improving its chances into the EU membership (not sure if there will be EU by the time they finish). The second is Iran also taking ME into its influence as a step into its way to be a major power player on the international seen, encouraged by its success in Iraq, its standup to the US- and west in general- plans in the region, and proceeding with its military program. A second layer of interest is GCC countries fearing Iran increased influence and wanting to offset that influence, and ME countries in general trying to stop the domino effect of this Arab spring.
It has been an established concept; you cannot have leadership in ME without holding the Palestinian resistance card in you pocket, and whoever takes the lead on this will be looked-up open by every citizen in all ME countries. Any country hoping or planning to expand into ME will need to appear as standing-up to Israel with its repression and injustice. Syria is very important to Iranian regime because it provide it with some legitimacy in ME by providing supply rote to Hizballh (fighting and winning against Israel) and its own ideological stand against Israel as part of the resistance. Also from Iranian prospective loosing Syria from the sphere of influence would symbolically mean loosing ground and this could mark the start of their retreat in the region.
Turkey had a great success in its economic policies next step has to be securing markets for its goods, and to cement its claim as new rising major power on the international stage, it needs to take a leadership role in ME and claim it as a strategical depth for the new power. One can see this in the recent clash and appeared conflict with Israel to win over people in ME, and be the champion of the Palestinian rights (trying to take Palestinian card by itself). At the same time promoting itself as the new successful model- Political Islam. Syria is first a great market for Turkish goods but most importantly it is a gate to ME market in general including GCC and north Africa. Second Syria is part of the national security for Turkey, having 1000 km borders and with large Kurdish population and small sources for its water supply.
This shows the high stakes politics in ME in general but more specifically the importance that Syria play at this point of time in the power play in ME. It is a zero sum game for both Iran and Turkey, they are fighting for the Palestinian card, and both planing to flux their mussels to claim influence over ME. Syrian regime was trying to balance things out between the two powers, and trying to benefit from the power struggle (to some success) but as the revolution weakened the regime and forced it to seek outside help, Turkey solutions included help on short term but with structural reforms that will lead eventually to the downfall of the current regime- by increased opining up and representation- where on the other side the Iranian solution was more accommodating to regime in Syria, therefore they chose to go with Iran. Iran came true to Syrian regime by providing technical, military and economical support (I still cant understand how the Syrian pound stayed stable all this long) and Turkey was left out as an enemy of the state in Syria. So far Turkey appear to step back and did not step-up to the play, so much so that EU and US started to stepping back in and neutralizing Turkey roll in the process. Iran did prove to be a reliable ally going all the way for regime inSYria while Turkey stand to loos most in this struggle, loosing prestige and economical interest, waving its dream of raising power even if Syrian regime falls later and Iran do not succeed.
In conclusion I ask the question; can Turkey afford to stay silent and the side, and in a bigger picture view, EU has issued a very harsh sanctions on Syrian regime, USA has sanctioned the regime with similar measures, can they really accept another failure after Iraq, especially a failure to Iran. Can the GCC stand in silence as Iran extent its influence to Iraq and Syria?
PS; i have not mentioned the sectarian card since i think it is means to accomplish strategy and not and end by itself.