The involvement of Trian is the latest sign of the growing clout of activists. While they have become increasingly powerful in recent years, forcing companies to do everything from buying back stock to selling assets, their ability to help bring about such a monumental deal represents a new high. Once viewed as fighters from the fringe who would be kept far from deal talks of two iconic companies, activist investors have won acceptance from some executives and boards, who now sometimes court their detailed analysis and thinking on decision making.