Pienaar joined the artillery branch of the Natal Police (NP) in 1911, and transferred to the Union Defence Forces (UDF) when they took over the NP in 1913. In World War I, he served as an artilleryman with the South African Overseas Expeditionary Force in German East Africa and Palestine.
Between the two world wars, Pienaar held various staff and command posts. He commanded
World War 2
In 1941-1942, during the North Africa Campaign, Pienaar fought in the battles of Sidi Rezegh and Gazala. On 10 March 1942, he was promoted to GOC 1st South African Infantry Division, which he led in the battle of Gazala, the retreat to Egypt, the defence of El Alamein, and the final battle of El Alamein. He was twice awarded the DSO and mentioned in dispatches twice for his service in North Africa.
On 20 November 1942 he was appointed Companion of the Order of the Bath (CB) "in recognition of the supreme gallantry and magnificent achievements of British and Dominion Troops and their Commanders in the present operations in the Middle East".
During the early stages of the North African campaign, the South African 5 Brigade had been destroyed on 23 November 1941, at Sidi Rezegh and on 21 June 1942, the 2 Division, with 4 and 6 Brigades under command, surrendered at the fall of Tobruk. As a result of these heavy losses Pienaar, now commander of 1 Division was cautious and increasingly reluctant to risk his troops, which lost the confidence of his British commanders. It was felt, a little unfairly, that his division now lacked the necessary aggressiveness and drive for offensive operations. However the division played an important part in the victory at El Alamein.
On the other hand, Ronald Lewin, who took part in the entire campaign, writes: "what is striking . . . is how often the British would squander a complete armoured brigade in some useless assault on a fixed position." Pienaar simply did not buy into the "Up Guards and at 'em!" approach of the British, which led to the decimation of the Commonwealth and Indian divisions that bore the brunt of so much of the fighting in the Desert War. "You know the three things I hate in this world— British lords, British generals and these bloody Guards!" he told Brigadier E. P. Scrubbs Hartshorn. "The burden of blame" for the loss of Tobruk in 1942, Churchill wrote in volume 4 of his memoirs, The Hinge of Fate, "falls upon the [British] High Command rather than on [the fortress commander South African] General [Hendrik] Klopper and still less on his troops."
Major General Pienaar was arguably one of South Africa's most charismatic and popular military commanders. An infantry regiment, exhibition hall at the South African National Museum of Military History and a suburb of his home town, Bloemfontein, were later named after him.
Sam Brewer, war correspondent for the Chicago Tribune, wrote in an obituary that Pienaar was "acknowledged by all the military authorities . . . as one of the best fighting leaders the British have found in this war. He was every inch a soldier and a man, and on top of that had a quality not always found in a tough General--he was loved like a father by his men. . . .
More than once he had hard words with higher authorities when he thought insufficient attention was being paid to the safety and comfort of the foot sloggers, who were bearing the brunt of the fight. Two points struck everybody who met Dan Pienaar--first his disregard for personal danger; second his solicitude for his men."