Hitler is under the "X". The Jack Russell was his. For another picture.
In World War I, it was common to find terriers in the trenches where they served three purposes: messengers (this was before radios, walkie-talkies and telephones), mortar detectors (a dog can hear an incoming shell long before a human can), and gas detectors (a dog can smell a drifting cloud of mustard or chlorine gas long before a human can).
The picture above shows one of the most famous and infamous veterans of World War I -- Adolph Hitler who was gassed despite the Jack Russell dog to the far right. Young Adolph is the fellow with the ridiculous mustache under the "X". In a later picture, Hitler insisted the dog be photographed sitting at his side.
Hitler's terrier apparently had been the mascot of English soldiers and ventured out into "No Man's Land" sometime in late January or early February of 1915 while chasing a rat. The dog jumped into a German trench where Adolf (himself a messenger) caught the Jack Russell terrier and decided to keep it. Hitler named the dog "Fuchsl" or Little Fox.
Adolph fed and taught Fuchsl tricks, and the Jack Russell terrier never left Adolf's side until August of 1917 when the dog was stolen at a train station, apparently by a railroad official who earlier that day had offered Hitler 200 Marks for the terrier. Hitler said he would not take 200,000 Marks for the dog. "I can look at him like I look at a human being," Adolph had written about the dog. "I am crazy about him."
In 1918, in an incident that might have been avoided had his terrier remained with him, Hitler was temporarily blinded by a British gas attack in Flanders.
Though Hitler's maniacal hatred, paranoia and obsession were already becoming self-evident during WWI, the loss of his dog and subsequent gassing may have contributed to his desire to scapegoat others for his -- and his country's -- failures and defeats.