When was the military tank invented?

There are designs dating back to Leonardo da vinci. The first practical use was in World War 1. They were used to break the stalemate in the trench warfare.

Da Vinci didn't design a tank. He designed a muscle-powered, moving shelter that ran on wheels. The idea goes back centuries before that. It was only once the caterpillar track and the petrol engine had been invented that building a true tank became possible. Military tanks (armoured fighting vehicles with armament, that run on tracks and can operate off-road) were designed in several countries in the decade before WWI - France, Austria, and Australia in particular - but not built. A prototype was built in Russia once the War began, but development went no further. It was in Britain and France that military men looked for a way of overcoming the stalemate on the Western Front and, after considerable experimentation, what we now call a tank proved to be the most suitable means. The problem is whether you mean "when was it thought of?" or "when was one made?" Both are difficult to answer, because lots of people imagined something that would do what a tank does, and several ideas came together (in both Britain and France) to result in tanks. It was a process. The best evidence we know for certain is that a Frenchman called Levavasseur offered the French Army something quite like a true tank in 1903.