Some people are comparing this to the late 30’s when Americans and other Europeans ran off to Spain to fight in the Lincoln brigades and others for the free (i.e. communist) Spainards against Franco’s fascist forces.
In some respects it’s true as you have a proxy war going on in Syria between the major players of the middle east. Just as the Spainish civil war was a proxy war. However, though the Americans that came back were sympathetic to socialist and even communist causes, none of them were looking at blowing up places in our country or carrying on the war here at home. Proof of that is none of them did. We let these American jihadis back into the states, I think we all know that bad things will follow.
Not a history expert here, but I think you are giving the Lincoln Brigades too much credit. Probably the major difference is that those guys actually had some intelligence and realized that they could destroy the country more efficiently by destroying its institutions from within.
And what we see now in so many sectors of society are the fruits of their efforts.