Viljoen served in 1 SAAF Squadron in May – June 1940 before being posted on attachment to 112 Squadron on 19 June 1940.

He left 112 Squadron in July 1940 and returned to 1 SAAF Squadron. This unit operated over Eritrea from July 1940 and was initially equipped with Gloster Gladiators.

In the beginning of October Captain Brian Boyle took a detachment of six Gladiators to Azaza, near Gedaref. Only six mechanics accompanied the detachment to this rudimentary base, where the pilots even had to prepare their own belts of ammunition for their guns. Early warning was almost non-existent, consisting of a native in the bush with a field telephone, whose only words in English were ”Aircraft over Gallabat”.

After an early warning on 4 October, Captain Brian Boyle led Lieutenants Viljoen and Andrew Duncan in a scramble from Azzoza. They were climbing over Metemma when they saw three CR.42s in line astern approaching them 500 feet higher. Climbing to meet them head-on, the Gladiators attacked and the six aircraft broke up into a dogfight. Lieutenant Viljoen shot down one Fiat in flames, the pilot bailing out. Captain Boyle’s aircraft was hit and damaged but despite this he continued to attack one of the Fiats, seeing it fall away apparently out of control, trailing smoke.
Subsequently the ground forces sent in a report that this second aircraft had crashed, but the Italian recorded the loss of only one aircraft (that destroyed by Viljoen), claiming in return one of the Gladiators shot down. It is possible that Boyle’s victim force-landed in a repairable condition, or managed to struggle back to base. Almost certainly their opponents were from the 412^a Squadriglia.

On 11 November Lieutenants John Coetzer, Andrew Duncan and Viljoen set off for Gallabat to intercept bombers. West of the town they ran into eight enemy fighters instead. These were stepped up in echelon in three groups, 1524 meters above the three Gladiators. Despite tactical and numerical disadvantage, the South Africans attacked, chasing two of the Fiats down to ground level before they escaped, while the rest made one pass on the Gladiators and then fled.
It seems that the Gladiators had clashed with Fiat CR.42s from the 412^a Squadriglia, which returned claiming one Gladiator shot down and two more as probables.
One Gladiator returned damaged by a single explosive bullet.

In December 1940 1 SAAF Squadron started to receive Hurricanes, which the unit operated together with the Gladiators until all of these aircraft was replaced in the end of February 1941.

At 13:00 on 16 December Lieutenants Leonard le Clues Theron and Viljoen and Second Lieutenant Jarvis scrambled from Azaza to intercept bombers. One CR.42 was seen, but it saw them early enough, outdistancing their pursuit in a dive.

On the morning on 27 January 1941, six Hurricanes and six Gladiators of 1 SAAF Squadron, using Sabderat as an advanced landing ground for refuelling, attacked Gura airfield during the day, many aircraft being seen there. While Major Wilmot, Lieutenant Theron and a third pilot remained above as top cover, the other nine strafed, claiming hits on nine S.79s, seven S.81s and three Ca.133s.
The Gladiators taking part in the strafe were led by Captain Brian Boyle (Gladiator N5815) and the other five pilots were Lieutenants Viljoen, Robin Pare, Andrew Duncan, John Coetzer and Taylor. Hurricane pilots included Major Wilmot, Captain K. W. Driver, Lieutenant John Hewitson and Lieutenant Leonard le Clues Theron.
After the strafe at Gura, Captain Driver and Lieutenant Hewitson continued to Adi Ugri where Driver strafed four more S.81s, claiming damaged to two of them. Hewitson meanwhile strafed other ground target.
The Italians reported that three S.79s and four S.81 were substantially damaged.

On 3 February six Gladiators from 1 SAAF Squadron flew forward to a new landing strip called ‘Pretoria’ early in the morning where they refuelled. Five of them (Captain Brian Boyle (N5824), Lieutenant Andrew Duncan, Lieutenant Robin Pare, Lieutenant Viljoen and Lieutenant H. P. Smith) took off again at 11:45 to strafe airfields in the Gondar area, to the south. A landing site was spotted to the south of Azozo on which five Ca.l33s were bombed up. The Gladiators attacked and claimed all five in flames. The Italians reported only one as actually destroyed in this attack.
Seeing CR.42s scrambling from Azozo, they flew over to investigate and Boyle, Pare, Viljoen and Smith claimed a shared damaged S.81 on the ground there when they reported that they had shot the wing off it while Lieutenant Duncan chased after a CR.42. The Gladiators were then attacked by Fiats and a big dogfight began. In the combat Captain Boyle claimed a Fiat shot down. Boyle reported that his claim was made towards the end of the dogfight and he was so close to the Fiat that he saw the enemy pilot clearly as he tried to bale out before crashing into the mountains.
Sergente Maggiore Enzo Omiccioli was shot down and killed in this combat. Reportedly he had scrambled alone.
The Gladiators then carried out a reconnaissance in the area, looking for reported Italian movements. On return, Lieutenant Smith crashed on landing at Azaza when starboard tyre burst and the aircraft slewed onto its side while Boyle suffered a flat tyre from the fight.

On 11 February Capitano Mario Visintini claimed a Hurricane over Keren. This claim was probably made in combat with Hurricanes from 1 SAAF Squadron, which had 11 aircraft on patrols over the area during the day. Two of them encountered three CR.42s, which dived away in thick cloud. Lieutenant Viljoen followed and failed to return. Running low on fuel, he had to land near a village, fortunately within territory in British hands. He obtained petrol, and attempted to take off next morning, but crashed. He returned to Agordat on foot. His aircraft was later recovered and repaired.
It seems that Viljoen had chased aircraft from 412^a Squadriglia, engaged in strafing British troops in front of Keren. Two of these fighters flown by Sergente Maggiore Lugi Baron and another pilot, force-landed at a forward landing strip due to bad weather.
Later on the day Visintini took off to fly back to guide these pilots home. It seems that during the flight he was however blown off course by high winds and while descending through clouds he crashed into the side of Mount Nefasit and was instantly killed.

Air reconnaissance had reported that new CR.42s being assembled at Massawa after delivery by transport aircraft, and at 08.00 on 21 February Major Lawrence Wilmot from 1 SAAF Squadron took off from Kassala with Captain Brian Boyle and Lieutenants Robin Pare, John Coetzer, Leonard le Clues Theron, Viljoen and E. A. “Tuppy” Jarvis. The Hurricanes refuelled en route at Agordat and took off at 11:00, attacking Massawa at midday. A small number of aircraft was seen outside the hangars, but the number inside was not known, so all six hangars were attacked, the pilots approaching at zero feet and firing directly into them; all were left in flames, the roof of one being blown right off by Major Wilmot. Six Ca.133s and a CR.42 were strafed in the open, and claimed in flames, while Lieutenant Robin Pare wiped out an anti-aircraft position and crew, which were firing on Major Wilmot. His guns then jammed but he continued to make dummy runs on the AA posts, being the last to leave the area. Lieutenant John Coetzer was however shot down and killed by AA fire, his Hurricane (V7658) crashing in flames.
In fact, three Ca.133s and two S.81s, all of which had been damaged the previous day, were destroyed, and one more of each was damaged.

He left the Squadron in 1941 but returned again on 18 August 1942. The unit was at this time operating in the Western Desert and equipped with cannon-armed Hurricane Mk.IIcs.

At 10:30 on 14 October, twelve Hurricanes of 1 SAAF Squadron provided top cover to 33 Squadron, escorting two tactical reconnaissance aircraft. Five Bf109s and MC.202s attacked, joined by three more. One Bf109 was seen to crash, and it had to be decided which of three pilots, Captain Viljoen, Lieutenant Smith, and Lieutenant Gilson, was responsible. All protested that it should be credited to the others, and eventually it was decided that it should be shared between them.

At 15:45 on 2 November, the Hurricanes of 1 SAAF Squadron took off with 213 Squadron as top cover, the 24 aircraft meeting 25 Ju87s with twelve Bf109s as close cover, about ten more being seen above these. The Ju87s were just going down to bomb when attacked, many of them being seen to have Italian markings. Captain Faure of 1 SAAF Squadron claimed one and a probable, Lieutenant Pryde one and a damaged Bf109, Captain Viljoen and Second Lieutenant Robinson shared one, and Lieutenant Smith damaged two. For 213 Squadron, Squadron Leader Olver claimed one, a probable and a damaged, Pilot Officer Carrick and Sergeant Usher one each, Pilot Officer Luxton and Sergeant James each a probable, and Pilot Officer Aitken a damaged.
Eleven Bf109s of I/JG 27 and ten from III/JG 77 had taken off at 15:35 to escort the Stukas, attacking ten fighters before bombing and 25-40 afterwards. Oberleutnant Josef Jansen of 2./JG 27 claimed one Curtiss fighter at 16:17 (possibly a P-40 from 3 RAAF Squadron flown by Sergeant Holder) and Oberfeldwebel Kaiser of III/JG 77 also claimed one.

From late 1942, he served as flight commander.

The Squadron re-equipped with Spitfire Mk.Vs from the beginning of November.

On 27 March 1943 he claimed a damaged Me210 30 miles north-west of Gabes.

He was posted away from the unit on 11 April 1943.

During 1943, he served as air fighting instructor at 73 OTU.

Viljoen ended the war with 1 biplane victory.

Claims:

Kill no.	Date	Time	Number	Type	Result	Plane type	Serial no.	Locality	Unit
	1940
1	04/10/40		1	CR.42 (a)	Destroyed	Gladiator II		Metemma	1 SAAF Squadron
	1941
	03/02/41	11:45-	1/5	Ca.133 (b)	Shared destroyed on the ground	Gladiator		S Azozo	1 SAAF Squadron
	03/02/41	11:45-	1/5	Ca.133 (b)	Shared destroyed on the ground	Gladiator		S Azozo	1 SAAF Squadron
	03/02/41	11:45-	1/5	Ca.133 (b)	Shared destroyed on the ground	Gladiator		S Azozo	1 SAAF Squadron
	03/02/41	11:45-	1/5	Ca.133 (b)	Shared destroyed on the ground	Gladiator		S Azozo	1 SAAF Squadron
	03/02/41	11:45-	1/5	Ca.133 (b)	Shared destroyed on the ground	Gladiator		S Azozo	1 SAAF Squadron
	03/02/41	11:45-	1/4	S.81	Shared damaged on the ground	Gladiator		Azozo	1 SAAF Squadron
	21/02/41		1/7	Ca.133 (c)	Shared destroyed on the ground	Hurricane		Massawa	1 SAAF Squadron
	21/02/41		1/7	Ca.133 (c)	Shared destroyed on the ground	Hurricane		Massawa	1 SAAF Squadron
	21/02/41		1/7	Ca.133 (c)	Shared destroyed on the ground	Hurricane		Massawa	1 SAAF Squadron
	21/02/41		1/7	Ca.133 (c)	Shared destroyed on the ground	Hurricane		Massawa	1 SAAF Squadron
	21/02/41		1/7	Ca.133 (c)	Shared destroyed on the ground	Hurricane		Massawa	1 SAAF Squadron
	21/02/41		1/7	Ca.133 (c)	Shared destroyed on the ground	Hurricane		Massawa	1 SAAF Squadron
	21/02/41		1/7	CR.42 (c)	Shared destroyed on the ground	Hurricane		Massawa	1 SAAF Squadron
	1942
	14/10/42		1/3	Bf109F	Shared destroyed	Hurricane IIc	798/AX-T		1 SAAF Squadron
	02/11/42		½	Ju87	Shared destroyed	Hurricane IIc	HL845/AX-N	El Alamein	1 SAAF Squadron
	1943
	27/03/43		1	Me210	Damaged	Spitfire V		30m NW Gabes	1 SAAF Squadron

Biplane victories: 1 destroyed, 5 shared destroyed on the ground, 1 shared damaged on the ground.
TOTAL: 1 and 2 shared destroyed, 1 damaged, 12 shared destroyed on the ground, 1 shared damaged on the ground.
(a) Almost certainly claimed in combat with CR.42s from the 412^a Squadriglia. 1 SAAF Squadron claimed two CR.42s for no losses while 412^a Squadriglia claimed one Gladiator for the loss of one of their own.
(b) The Italians reported only one as actually destroyed in this attack.
(c) Three Ca.133s and two S.81s, all of which had been damaged the previous day, were destroyed, and one more of each was damaged.

Sources:
Dust Clouds in the Middle East - Christopher Shores, 1996 Grub Street, London, ISBN 1-898697-37-X
Fighters over the Desert - Christopher Shores and Hans Ring, 1969 Neville Spearman Limited, London
Springbok Fighter Victory: East Africa Volume 1 1940 – 1941 – Michael Shoeman, 2002 African Aviation Series No. 11, Freeworld Publications CC, ISBN 0-958-4388-5-4
Those Other Eagles – Christopher Shores, 2004 Grub Street, London, ISBN 1-904010-88-1