Following the outbreak of the First World War, Dinesen attempted to enlist in the British, French, and United States armies, before finally being accepted by the Canadian Corps in 1917.

[3] During the Battle of Amiens he was 26 years old, and a private in the 42nd Quebec Regiment (Royal Highlanders of Canada) Battalion, Canadian Expeditionary Force when, on 12 August 1918 at Parvillers, France, he displayed conspicuous bravery when, five times in succession, he rushed forward alone against entrenched enemy troops and put hostile guns out of action.

On 7 August 1918, whilst waiting to go into action, Dinesen wrote of his night in Gentelles Wood: 'All day long we rested in this pleasant spot – we even had permission to make a little fire here and there under a thickly branched tree and do a bit of cooking.

The regular meals are good and plentiful, of course, but we never miss a chance of eating unlimited quantities of extra food.

According to Blixen, she had inadvertently alienated many of her neighbors by helping to buy horses for a German officer she met on board ship while sailing to Kenya for the first time.

Only months later this officer, General Paul Emil von Lettow-Vorbeck, was named commander of the German forces in East Africa, and waged an effective campaign against Blixen's English neighbors.

[8] In 1918, Thomas Dinesen moved to British East Africa to help his sister manage her coffee farm in the Ngong hills southwest of Nairobi.