Considering that Syria initially kicked off because of the Arab Spring I would say that our direct involvement in the start of the conflict was probably a lot less than WW2. It was well underway by the time we actually got involved in Syria. We actually refused to take part in air strikes initially when they were just against Assad. It's not like we invaded and started the conflict.
And, actually, we did bomb Germany. And all Germans were banned from travelling here including Jewish people during WW2 and many German people were interned for national security. And as I said, a comparable situation would have been if we had open doors to anyone from Germany throughout the war, as long as they could get here. No questions asked about whether or not they were Nazis or held views which supported the persecution of Jews/Communists/Homosexuals/Gypsies/ the disabled etc or were hostile towards non-Nazi states.
IMO there is a big difference between taking members of a minority group who are explicitly being targeted for persecution (in this case, Christians, Yazidi, some Shia; in the case of Germany, Jews, Gypsies, gays etc) and taking large numbers of the majority group doing the oppressing without actually bothering to ask hard questions about who they are and what they believe.
If you give asylum to those who are doing the oppressing, and they continue with their oppressive behaviour in a new country, then you haven't actually given anyone asylum. You've just let the violence spread and put greater numbers of people at risk.
Examples of this recently include Christians and Yazidi being persecuted by Sunni Muslims in German refugee camps:
www.christiantimes.com/article/muslims-threaten-christians-and-other-minority-groups-in-german-refugee-camps/55325.htm
And Christians being thrown overboard on boats:
www.theguardian.com/world/2015/apr/16/italian-police-arrest-migrants-alleged-to-have-thrown-christians-overboar
The head of the Syrian Orthodox Church has expressed concerns about the treatment of it's members in Europe, including violence, sexual violence threats and intimidation.