Martin Sorrell, chairman and CEO of WPP Group PLC, has enjoyed a bird’s-eye view of the changing logic of global competition. Back in 1986, he left the high-flying British ad agency Saatchi & Saatchi, bought a shopping-cart manufacturer — Wire and Plastic Products PLC — and turned it into a vehicle for assembling one of the world’s most powerful advertising conglomerates. He acquired J. Walter Thompson and Ogilvy & Mather. He bought the public-relations giant Hill & Knowlton and moved his company into management consulting. Today, WPP is a marketing-services colossus, with 33,000 employees, revenues of $3.2 billion, and offices in 92 countries. Its clients include several of the world’s most powerful companies — Ford, IBM, Unilever — and its operations affect some of the world’s best-known brands.