1st line... "Princess Cadence needs our help, her magic will not last forever."
Her main spell is the love spell, which was used to end an argument in pt1 of the wedding episode. With that fact, plus the music line, it can be perceived that Cadence is in the Crystal Kingdom, using her magic to prevent fighting.
Now, the "Crystal Fair" is obviously going to be used to restore Crystal patriotism. Now here's the question that I want answered, Why are the Crystal ponies fighting?
I'm also beginning to wonder if this is tied to "I Wasn't Prepared For This" maybe Twilight was assigned to prevent a war between the Crystal Ponies, a task that could easily make her doubt her abilities.
What does everypony else think?