Stewart's motive for alienating Macy's is simple: Ron Johnson said in an email that she could earn $500 million if the deal went well.

Johnson made it clear that he needed JCPenney to execute a turnaround of his brand. Losing the line would be a devastating blow to an already-struggling retailer.

Macy's, which has had a deal with Stewart for several years, sued her company for breach of contract shortly after she announced the JCPenney line. The department store also sued competitor JCPenney.

Here's what's at stake for the three biggest players in the trial: