What I did: I was playing World of Tanks, and I wondered how the Tanks came about.
What I learnt: I learned that during the First world war, also known as the war of the trenches, tanks were used as breakthrough vehicles to break through a stalemate between two opposing trenches. The heavy armour tanks sported were impenetrable by even 50' calibres fired by heavy assault rifles. The British were the first to develop these machines, but during the second world war, the Germans perfected these designs when they created the Panzer series which sported massive calibre guns. The British meanwhile developed the Churchill series which had thick armour. The Russians combined both in the Isof-Stalin and KV series the former had large calibre guns and heavily sloped armour which caused shells to bounce off or ricochet, the later had an unreal amount of thick armour and solid guns. In response to these Tanks, the Tank Destroyer was created. Some were thick armoured behemoths like the German Jadgpanzer E-100, others were lightly armoured but fast like the Russian Object-268. All of the Tank Destroyers had massive calibre guns that stretched from 10cm guns to 17cm guns or even the massive Naval Gun, the 18.5 cm gun.
Summary: Tanks sported an arms race to make tanks that were even thicker armoured of had massive amounts of sloped armour and guns that could penetrate these forms of armour.