WASHINGTON—The Justice Department on Thursday sued to block the tie-up of Hewlett Packard Enterprise and Juniper Networks, a $14 billion deal that would join the second-largest and third-largest providers of wireless networking solutions for large corporate customers.

The Justice Department sued to block Hewlett Packard Enterprise's $14 billion acquisition of rival Juniper Networks on Thursday, the first attempt to stop a merger by a new Trump administration that is expected to take a softer approach to mergers.

DOJ cited reduced competition in the wireless market as the biggest problem with the proposed $14 billion sale of Juniper Networks to Hewlett Packard Enterprise. The vendors say they plan to “vigorously defend the transaction in court.