Fifteen years ago the United States declared war against Iraq, because it wanted to remove its then-leader Saddam Hussein from power. Iraq fabricating and owning weapons of mass destruction was used as one of the excuses for this, with little to no evidence.
Almost everybody opposed this war. There were massive protests, especially from the left, against this war. Not just in the United States, but all over Europe. I still remember public transport being delayed because of protest marches here in my city (in Finland).
Very recently, the United States is attacking and is probably about to declare war on Syria, because it wants to remove Bashar al-Assad from power. Alleged chemical weapon attacks, allegedly by the Syrian government, are being used as an excuse for this, with little to no evidence. (There is no definitive evidence that any such attack has taken place at all, and isn't just a fabrication by Isis. And even if the attack did happen, there is a ton of evidence that it was most probably perpetrated by Isis, not by the Syrian government.)
Where are the anti-war protests now? In fact, the attitudes seem completely different, especially from the leftist media, and leftist politicians. Ironically, it's the right-wing media, especially in the United States, that's opposing this war, while the left-wing media is mostly supportive (which is pretty much a complete reversal of what the situation was 15 years ago with the Iraq war.) As incredible as it sounds, many right-wing American news media and social commentators are opposing the actions of their own elected president on this matter.
One has to wonder what exactly is the difference here. After all, both incidents share quite many similarities. So many similarities, in fact, that reason would tell that the attitudes should also be similar. Yet, the attitudes seem to be complete reversed.
There are three differences that come to my mind as possible affecting factors:
Firstly, al-Assad's government is mostly secular. Hussein's government was deeply Islamic.
Secondly, al-Assad and his government has been fighting against Isis (with great success so far), and has mostly a positive view of the west. Hussein was a deeply religious Islamic and pro-Islam dictator who mostly oppressed dissenters, and who had a negative view of the west.
The western regressive left-wing ideology has this extremely strange and bizarre attitude towards Islamic extremism in general, and Isis in particular. On one hand they nominally condemn terrorist extremism, but on the other hand they will defend it against "oppressors", especially ones that seem to be in friendly relationship with the west, and have western values. It seems to be a strange struggle between opposing terrorism and oppression, and self-hatred and hatred of our own western values and culture.
Thirdly, the Iraq war did not cause a massive flood of economic migrants into Europe. The Syrian situation did. And to the regressive leftist cult, a massive flood of brown-skinned migrants entering Europe is one of the greatest things ever, and thus they will, seemingly, even support the United States going into war, if it will keep the floodgates open.