What if it's a very bad deal?
Allow Russia and Iran to take over Syria. Break off deal with Iran. Result: refugees never go home; MORE millions flood the First World.
Which, as a current Atlantic article
http://www.theatlantic.com/…/syria-aleppo-assad-rus…/462231/ points out can enhance the appeal of right wing parties in Europe. Though Europeans may be able to deflect a great deal of the refugee flow to Brazil, which so far, has something of an open door to Syrian refugees and much more room to settle in, though it has fewer social benefits and often workers in frontier states work under slave labour conditions).
We need to face the fact that Sunnis and Kurds are no longer able to live in multi-ethnic states in the Mideast. Either borders get redrawn and independent states created or millions of Sunnis and Kurds will have to find new homes elsewhere in the world. If neither is possible, the result will be genocide, just as it was for the Armenians 100 years ago in the same places. When people in the Mideast rebel against a subordinate place in society, unless they win hands down (as they did in Syria when the Alawites took over) they no longer have a place. And no intervention by outside Great Powers can change this.
Rebellions continue to break out.; no peace restored. More rebels join the IS to fight Russia and Assad. IS takes more territory, at least for a while. More terrorists come there to join the fight. Turkey attacks Russia/Assad and the Kurds too. The USA no longer looked to as peacemaker of the Middle East; Egypt and Iraq become Russia's allies.
So let Russia have another Afghanistan on it's hands. Syria and Iraq (along with Yemen) are going to be an Afghanistan for somebody. Frankly, the only nation which MIGHT be able to commit enough troops (if so inclined) to put a complete damper on the Mideast is China.
Israel is moving closer to Russia and China too. Selling desal technology to China. Agreeing to allow Gazprom to market it's natural gas for export. Even Israel can make it's peace with a dominant Russia and China if it comes to that. Unlike some countries, Israel is only in this world to survive and remain independent.
Iran resumes rapid nuclear development. Israel attacks Iran; USA joins in, war with Iran results. Russia defends Iran; world war results.
Trump breaks NATO treaty commitments, or winks at Putin that he won't fulfill them. Result: Russia steps up attacks on Ukraine and moves into the Baltic. Russia emboldened to go even further, and so is China. World War results, with USA and its allies in a weaker position than today.