When Havas and Horizon Media launched Horizon Global on Monday, an independent joint venture between the two rival agency groups, it was a sign of the times.

The tie-up underscores the pressure facing mid-tier networks in a consolidating market. Omnicom’s $13.5 billion acquisition of IPG, expected to close before the end of the year, will narrow the field of global media giants while creating a new largest player. Publicis began consolidating its media business more than a decade ago. Meanwhile, WPP is restructuring its media arm by merging operations across its agencies, and the future of Dentsu’s international business hangs in the balance .

Horizon and Havas—each strong in their home markets but smaller on the global stage—see the joint venture as a way to compete for global m

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