2000 years old and are still poor countries.
India was one of the richest countries in the world, before the Islamic invasion began in the 12th century. The invaders looted most of the treasures (such as gold, precious gems and artworks) and destroyed the major cities. The intellectuals and other important sections of the society such as physicians and scientists were disproportionately targeted.
As the people become poorer, the country has to stretch their money to assist them and that drives the system down further. and same happened with India.
Well, the people became poor as a result of the foreign invasions. There were a number of Islamic invasions in North India, killing hundreds of thousands of people every time. The major temples were looted, and the treasures were stolen.
You are onto something, political instability is partly responsible for poverty.
However, not, if you take 300, 400 or 500 years into account. South Korea was for much of 20th century either occupied, at war or ruled by dictators, yet today its living standarts are broadly comparable to Germany.
Rather I propose model where resources per people (ie. overpopulation), local work ethics aswell as geopolitics all play a part in overall scheme of things.