There are a number of factors involved.
Israel wants to stop Iran getting a nuclear bomb and there has been talk over teh years of a strike on Iran. Netanyahu addressed the Senat just weeks ago and reiterated that Iran should not be allowed to develop bomb.
Saudi Arabia wants Iran curbed because its Shia Islam is a potential force against Saudi's strict Wahhabi Sunni Islam.
Then there is oil. In about 2010 Iran, Iraq and Syria agreed to make a pipeline so that oil and gas, I think, would get to Europe via Syria. This was not in the interests of Qatar who have their own huge fields and want to supply Europe.
In about 2010 the Syrian rebels started opposing the Assad regime. They were funded by some of our allies to topple Assad. Syria is supposed to fall so that it weakens Shia Islam before any possible attack on Iran.
Russia and Iran do not want war and Russia wants to protect its naval base interests in Syria etc.
The Free Syrian Army has been helped which is why the war has lasted 4 years.
"Update: U.S. training Syrian rebels; White House 'stepped up assistance'
CIA and U.S. military operatives have been secretly teaching Syrian rebels how to use anti-tank and anti-aircraft guns, the Los Angeles Times has learned. The White House refuses to confirm but says 'substantial assistance' is being provided."
articles.latimes.com/2013/jun/21/world/la-fg-wn-cia-syria-20130621
Against all expectations and against all the billions that have been thrown against Assad by some forces in Saudi Arabia, Qatar and Turkey, Assad has survived for 4 years. In fact, Assad is winning and the Free Syrian Army needs lots of help. Bombing Assad would have helped the Free Syrian Army but for some reason Cameron failed to get it through and did not try again and just dropped it. The US military does not seem to have pushed to do it and it was dropped by Obama.
Isis, Al Nusra and all teh rest of them were aided by Sunni forces to help try and topple Assad. But against all the money and forces thrownat him, he is still winning.
Russia helped him as did Hezbollah who probably turned teh war against Al Qaeda, Al Nusra, Isis and all the rest of them.
Some forces were angry that Russia managed to stop the plan and now we have a conflict for Russia on its doorstep in Ukraine.
The Saudis are unlikely to really sort Isis out because Isis is a Sunni Jihadi bunch of mercenaries who mainly attack Shia Muslim areas.
The Saudis have not committed ground troops to defeat the Isis Jihadi mercenaries, Al Nusra, Al qaeda and all the rest of them, but we hear that they may be about to send ground troops into Yemen to stop the Shia Houthi uprising
"As Saudi Arabia began pounding the rebels with airstrikes, countries from the Middle East to Pakistan were said to be prepared to commit troops for a ground assault."
Time Magazine reported that there is a feeling in Iran that ISIS is a Western creation
time.com/3720081/isis-iran-us-creation/
The battle for resources and influence across the planet is heating up primarily due to teh disastrous financial banking system and the possible beginnings of teh end of dollar hegemony. If the dollar loses world reserve currency status, the impact on the US and the Western financial and banking system will be massive.
The rise of the BRICS bank as a challenger to the IMF means that countries will have a choice from whom to borrow.
The US wanted Britain not to join the Chinese led Asian Investment Bank but Britain was the first Western country that did and now others have followed and the US is joining it as well.
The financial system is in trouble. If there is a Western backed Maidan style coup in Russia or Russia is busted up and looted then the system will buy time and probably be saved.
But whatever happens, the world is likely to get more dangerous in the coming decade or possibly sooner as the financial system breaks apart and the BRICS bank poses a challenge to IMF, World Bank etc financial hegemony.