Both major party candidates claim to favor creating safe zones in Syria. John Ford describes some of the many problems with this idea:

Matters are further complicated by how rebel forces would react to the creation of safe zones. Areas of Syria protected from the Assad regime by western military forces would attract rebel groups, who would seek to use these areas as shelter from the fighting or to establish bases from which they could launch attacks without fear of reprisal. If rebel groups gather in the safe zones it would only increase the risk that Assad would try to launch attacks into safe zones. This would increase the need for U.S. military forces to protect safe zones – a commitment that could require 30 thousand ground troops according to some estimates.

None of this would necessarily be insurmountable were it not for the presence of Russian military forces in Syria.

Since Russian forces are present in the country, that makes the creation of safe zones potentially even riskier than it would otherwise be. Retaliating against Syrian government forces that attack a safe zone runs the risk of killing Russian personnel with all of the dangers that come from a crisis with Russia. Even if Russian forces weren’t there, a safe zone would still require a substantial number of soldiers on the ground to protect it, and very few governments would be willing to put their soldiers in such an unenviable position and given such a thankless task. In order for safe zones to remain secure, they would have to be kept as neutral sites so that anti-regime groups couldn’t exploit the protection afforded to them to launch attacks, but the many governments that back various rebel groups would not accept that condition and would try to have it both ways. Civilians would likely still come under attack, but in the process the U.S. would also be drawn deeper into the conflict.

Clinton has said many times that she won’t send U.S. ground troops into Syria, and she has also said that she wants to establish safe zones. It is extremely unlikely that the latter can be done without also doing the former, but because Clinton has largely been given a pass on her foreign policy views and because foreign policy as been so neglected in this campaign the public has rarely heard anything about this. In order for safe zones to do what they are intended to do, the U.S. would have to risk conflict with Russia, as Ford says later:

Safe zones have a superficial appeal to western policymakers who want to protect civilians but are afraid of the consequences of deeper engagement in Syria. The appeal is illusory. True safe zones would create the risk of a wider war with Russia.

Calling for the creation of safe zones seems innocuous enough, and it also allows politicians to claim that they are in favor of “doing more” in Syria without appearing to be overly aggressive. Clinton is on record supporting this measure, but has never acknowledged what it would require. The reality is that carving out safe zones in Syrian territory is a very aggressive measure that requires a substantial military commitment and entails significant risks of escalation with a major power. On top of all that, these proposed safe zones aren’t going to be successful on their own terms without courting unacceptably high risks of an expanded war.