In 1904, he was commissioned into the British Army's Bedfordshire Regiment having spent the two years prior at the Royal Military College at Sandhurst in Berkshire.

The battalion was shortly dispatched to the Western Front, arriving at Zeebrugge in early October 1914, as part of the 21st Brigade, 7th Division, and fought in the First Battle of Ypres later that month.

[4] Foss' battalion advanced to the northwest of Neuve Chapelle on the opening day of the battle, 10 March, in support of the Royal Scots Fusiliers to the east.

They held their position for the following day but on 12 March the neighbouring Royal Scots Fusiliers had to fend off an attack on their trenches by the Germans.

Foss led a group of men with handheld bombs on a flanking raid and was able to recapture the lost trench.

After the enemy had captured a part of one of our trenches, and our counter-attack made with one officer and twenty men having failed (all but two of the party being killed or wounded in the attempt), Captain Foss, on his own initiative, dashed forward with eight men, under heavy fire, attacked the enemy with bombs, and captured the position, including the 52 Germans occupying it.

Of the men who accompanied Foss during his attack, several were recognised with gallantry decorations; one, Private William Eade, was awarded the Distinguished Conduct Medal and the Order of St George.

Sergeant William Peggs was also awarded the Order of St George while a third man, Private Walter Scrivener, killed the day after Foss's action, was mentioned in despatches.

[2] In 1918, he instructed at a staff school in Cambridge before returning to the Western Front shortly before the end of the war with a British infantry corps.