Personally I never really understood it entirely before
In essence the official reason is: We’re fighting the turrurists (aka ISIL, Isis, Daish).
While the actual reason is: We’re actually arming and backing exactly them, in the fading hope of toppling the democratically elected Assad government who we don’t like.
Which makes sense.
At least in context of everything else we’ve learned about the Empire in the last couple of years.
Wave the old glory. Bless the troops. Exercise your freedoms and duties as a citizen.
Do all you can to keep believing that your side is, that We are, still, still, still The Good Guys.
Let your illusions last until they shatter.
Live the Good life while it lasts.
Judgment Day is nigh.