You’ve read all of your free items this month. To register, get added benefits and unlimited access to articles, Become a Member. Already a Member? Sign in. Advertising Age Article Abstract Martin Sorrell Warns WPP Employees to Stay On Guard After Merger Collapse Beware of 'Wounded' Publicis and Omnicom Execs, Writes WPP Chief in Internal Memo By Shareen Pathak, Published on May 12, 2014 1 Want more Access, Content & Connections? Upgrade to Membership. We are glad you are enjoying Advertising Age. To get uninterrupted access and additional benefits, become a member today. Become a Member Already a member? Log in.