McDonald's has identified its next big worry: Wendy's. "This is the face of the enemy over the next 18 months," correspondence recently distributed to McDonald's Corp.'s franchisees asserts, in highlighting a series of potential problems. The reasons behind that conclusion, as laid out in a document that Dow Jones Newswires reviewed, range from Wendy's International Inc.'s forthcoming entry into the breakfast business _ McDonald's most profitable area _ to a battle over chicken sandwiches and to the belief that Wendy's intends to vigorously pursue young customers.