Caldwell began the war in the New Zealand Territorial Infantry. Having already learned to fly, he traveled to England to join the Royal Flying Corps in April 1916. Following flight training, he was assigned to 8 Squadron in France on July 29, 1916. After scoring his first victory with a B.E.2d, he was reassigned to 60 Squadron in November of 1916. By September of 1917, "Grid" shot down seven enemy aircraft with Nieuport Scouts, then began flying the S.E.5a with which he scored his ninth victory on 15 September. Returning to England in October, he served as an instructor before returning to France in April of 1918 as commanding officer of 74 Squadron.

Before the war was over, Caldwell survived a mid-air collision and scored sixteen more victories. But for his poor marksmanship, some thought that "Grid" might have been one of the highest scoring aces of the war.

Caldwell eventually returned to New Zealand where he became a farmer and married the sister of Frederick Gordon. During World War II, he served with the Royal New Zealand Air Force, attained the rank of Air Commodore and was awarded the CBE.