The outbreak of war found Australia with no modern tank force and little industrial infrastructure. The AC1 Sentinel was a home-grown tank developed at lightning speed to fight off the anticipated Japanese invasion.

An AC3 tank.

In spite of the speed with which it was produced, the AC1 Sentinel was a remarkably innovative design featuring an all-cast hull and a heavy armament. This is the AC4, which mounted a 17-pdr gun.

In 1939 Australia’s armed forces had virtually no modern tanks and lacked almost any form of heavy engineering background to produce them; even an automobile industry was lacking. Nevertheless the Australian government realized that it was unlikely that any large amounts of heavy war matériel would be available to Australia from overseas, and so set to produce its own. Among the requirements were tanks, and as there was no local expertise on the subject one engineer was sent to the United States and an experienced engineer was obtained from the United Kingdom.

With this experience to hand the Australian army staff issued a specification and Australian industry set to with a will, The first design, known as the AC1 (Australian Cruiser 1) was to have a 2-pdr (40-mm/l.57-in) gun and two 7.7-mm (0.303-in) machine-guns, and it was decided to use as many components of the American M3 tank as possible. The power plant was to comprise three Cadillac car engines joined together and extensive use was to be made of cast armour. A second model, to be known as the AC2, was mooted, but by late 1941 as the Japanese became increasingly aggressive in the Pacific, the AC2 was passed over in favour of the existing AC1, which had armour ranging from 25 mm (1 in) to 65 mm (2.55 in) in thickness.

The first AC1s were ready by January 1942 and were soon named Sentinel. The whole project from paperwork requests to hardware had taken only 22 months, which was a remarkable achievement since all the facilities to build the tank had to be developed even as the tanks were being built. But only a few AC1 tanks were produced as by 1942 it was realized that the 2-pdr gun would be too small to have any effect against other armour and anyway, the hurried design still had some ‘bugs’ that had to be modified out of the design. Most of these bugs were only minor, for the Sentinel turned out to be a remarkably sound design capable of considerable stretch and modification. This was just as well, for the Sentinel AC3 mounted a 25-pdr (87.6-mm/3.45-in) field gun barrel in the turret to overcome the shortcomings of the 2-pdr.

The 25-pdr was chosen as it was already in production locally, but it was realized that this gun would have only limited effect against armour and the Sentinel AC4 with a 17-pdr (76.2- mm/3-in) anti-tank gun was proposed and a prototype was built. This was during mid-1943, and by then the background to the hurried introduction of the AC1 into service had receded. There was no longer the chance that the Japanese might invade the Australian mainland and anyway, M3s and M4s were pouring off the American production lines in such numbers that there would be more than enough to equip all the Allies, including Australia, Thus Sentinel production came to an abrupt halt in July 1943 in order to allow the diversion of industrial potential to more important priorities. The Sentinel series was a remarkable one, not only from the industrial side but also from the design viewpoint. The use of an all-cast hull was way ahead of design practice elsewhere, and the ready acceptance of heavy guns like the 25-pdr and the 17-pdr was also way ahead of contemporary thought. But the Sentinel series had little impact at the time for the examples produced were used for training only.

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