Iran has been a staunch supporter of Assad for a very long time and that will never change. Their network of "resistance" that they've built over the past decades is too important to ever abandon. Through Assad and Syria Iran has its supply line to their front line with Israel in southern Lebanon via Hezbollah. They've spent hundreds of millions of dollars building infrastructure and supplying weapons and training to Hezbollah. This is why Iran will continue to pump weapons and money into Syria in unwavering support for Bashar Al Assad. It is not for their love of the Syrian people but it is a life line in their never ending fight against Israel and the Sunni gulf Arab States.
The rise of the Islamic State has brought a new foe into the game for Iran. This is an enemy that is all too close to Iran's doorstep. Iran has openly engaged ISIS in Iraq through air strikes and support of Shia militias. They have not hid this at all. They've sent their top general Qassam Suilemani to direct the Shia militias in some key battles that forces ISIS to flee key towns such as Amerli. As the west tries to keep combat troops out of the fight and relies on air strikes Iran will do everything possible to keep ISIS from getting any closer to their border.
Recently ISIS has become engaged in fighting on the border areas of Lebanon and have been gearing up for an assault in Lebanese territory for the town of Ersal. This could lead to some very interesting developments as this morning a Lebanese parliament member made a statement that Israel won't allow ISIS to occupy southern Lebanon.
Could an ISIS attack on southern Lebanon bring together strange bedfellows? Southern Lebanon is the home of Hezbollah and key to Iran. Southern Lebanon also sits on the border with Israel who certainly will not sit back and watch another Al Qaeda offshoot who seems their destruction take up residence on their northern border.
The possibility of an Israeli/Iranian/Hezbollah alliance is slim. The west wants to keep Israel out of this fight for good reason but Israel has proven she will not stand still if threatened. So what are the likely scenarios?
Most likely: The U.S. and allies will provide weapons and maybe even air support to the Lebanese army. Who in turn would team up with Hezbollah on the ground. Iran will deploy weaponry and generals as well to oversee the fighting.
Less likely: Israel launches air strikes against ISIS positions. This is not likely because Israel would like to sit back and watch ISIS and Hezbollah degrade each other. Israel would only act if their is a direct threat or if advanced weapons enter Lebanon. She will be watching closely but restrained.