GWADAR, Pakistan—A Pakistani brigade of 2,000 soldiers guards a small contingent of Chinese workers here from the threat of jihadists and separatists, reflecting the challenge of turning this remote fishing town into the hub of an economic corridor between the two nations.

Developing Gwadar would give China a new trade link from its relatively undeveloped west to key Arabian Sea shipping routes at the mouth of the oil-rich Persian Gulf—giving it potentially strategic as well as economic leverage.

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