Final answer:
Bolivia lost its Pacific coast to Chile in the War of the Pacific.
Step-by-step explanation:
The country that lost its Pacific coast to Chile in the War of the Pacific was Bolivia.
The war started in 1879 when Chile declared war on Bolivia and Peru in order to gain control over valuable mineral deposits in the region, particularly nitrate and guano.
As a result of the war, Bolivia lost its access to the Pacific, and its territory was incorporated into Chile.