At the outbreak of war in 1939, Great Britain was only at the early stages of rearmament. Nowhere was this more evident than in tank design and production.
The economic woes of the inter-war period allied to the still vivid memories of the First World War limited both the ambitions and the capabilities of the tank protagonists. Plus there was a doctrine which fatally flawed tank design: the decision to pursue two categories of tank; heavily armoured but slow ‘infantry tanks’ and more agile, but lightly armoured ‘cruiser’ tanks, splitting the effort and resources.
As if that were not enough, the heaviest gun mounted was only a 2 pounder (40mm) and many tanks were still only armed with machine guns. Although the very first encounters in France in 1940 revealed the problems, the lead times for change meant that it was years before British troops received a truly effective tank.
Forbidden by the Versailles Treaty from building tanks, Germany nevertheless pursued secret experiments in Russia and elsewhere, so that by 1933, with the rise of the Nazis and the start of a major rearmament, they already had a clear idea of what they needed. Tactics drove the development. Guderian formulated the theory of the armoured attack and the groundwork for ‘Blitzkrieg’ was laid.
The early Panzer I and II were really only viable for training purposes, but they had the virtue that they could be built relatively quickly to provide imposing numbers. The tanks which were to become the mainstay of the Wehrmacht throughout the war, the Panzer III and IV started life in the mid-thirties and had a long development process aimed at achieving the optimum design while allowing for future improvement.
On top of this, after the invasion of Czechoslovakia brought the considerable Czech industrial resources into their grasp, the Germans inherited the very serviceable T35/T38 tanks, which they lost no time in putting into service. With only a relative handful of the better Mark IIIs and IVs the German forces which overran Poland and then France were equipped with Panzer Is, IIs and T38s.
In the inter war period, the USA invested little in tank development, for the same reasons as Great Britain.
They too had split their focus between heavy tanks and light or cavalry tanks with the same result. Observing closely the course of the war in Europe in 1939 and 1940, they started to rectify this with great speed. They accelerated the programme to develop a 75mm gun-armed medium tank, producing the M3 Grant and the excellent M4 Sherman almost in parallel.
At least as important as the basic design was the way in which American industry was geared up to mass produce these tanks, enabling the USA to arm not only its own army but large parts of all other Allied armies. With excellent reliability thanks to proven components, the M4 was the equal of the Panzer III and early IVs, but was outclassed by the Panthers and Tigers appearing after 1943.
Development of anything heavier had been neglected, so a series of Sherman developments sought to fix the problem as far as possible while heavier tanks were hurriedly developed.