advisors will be placed in another country they are generally referring to Special forces Troops. SF troops are tasked with organizing the indigenous people of a country into an Army which is capable of creating havoc and just being an overall nuisance to their enemies. They are expert trainers and can mobilize a small force with primitive weapons into an efficient fighting machine.
This places them in the category of what the military calls a “Force Multiplier.” They add to the overall campaign strength in unpredictable ways. They probably won’t win the war if left alone but they will give the enemy a major migraine. When their actions are synchronized with the actions of other military units in the area, they will add greatly to the overall effort.
In the case of Syria they are assisting the Kurds who live in the Northern part of the country in the fight against ISIS. I imagine that they have been there much longer than we are aware of because the US military does not advertise their presence in conflicted areas.
As advisors their primary role is not to join in the battle but they will if they feel that it is necessary. It isn’t prudent for them to join the fighting for several reasons. The most important is that it is difficult to oversee the fighting of your trainees if you allow yourself to also become embroiled in the action. They take on great risks as advisors because they are armed and present in the battle.
Many have questioned the commitment of the United States in the battle against ISIS, but I feel that it should be done exactly as it is being handled right now. We are allowing the residents of Syria to handle their own problems with the most support that we can give short of sending ground troops, while keeping the majority of our fighting men out of harms way.