PHOTOGRAPH BY UNDERWOOD AND UNDERWOOD, NATIONAL GEOGRAPHIC

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A French sergeant and his dog wear gas masks on their way to the front lines in World War I. With their sensitive noses, dogs were able to recognize gas attacks and bark a warning to troops in the trenches before humans could smell the chemicals.

PHOTOGRAPH BY UNDERWOOD AND UNDERWOOD, NATIONAL GEOGRAPHIC

Dog Held Hostage by Taliban Part of Long Line of Combat Canines

For centuries, dogs have gone to war. But it's rare to hear of a canine held hostage by the enemy.