There are many ways to destabilize and corrode a country apart from the sheer muscle power. There are better subtle ways to do so which involves not your men but the men of the enemy and their own resources. The best way is the deceptive way as suggested by the history of all great fallen empires.
The world's biggest brain drain is happening in India. Almost all the top brains of India are being wooed abroad to serve foreign countries and foreign companies. majority of the NASA scientists are Indians, Majority of the Microsoft and many other top 500 MNC companies are Indians. In USA, Indian students are academically way ahead than any other ethnic group. Indian ethnicity has the highest average income compared to Jews or any other community. Imagine all this talent was in India but not anywhere else. India would have been a superpower which is against the interest of many countries who are in the top of the chain of the world politics and power. So it is in their interest to make sure India doesn't have a favorable environment for the talented but remains a last choice for the intelligent and talented. It is in record that when bill gates was asked why is he outsourcing jobs to India and hiring Indians in his company but not just Americans, he said "If I don't outsource jobs to India or Indians then they will form their own Microsoft". This was proved when USA refused to give uranium to India for us to be a nuclear powered nation we developed our own nuclear warheads using thorium in a mere 2 years of time. Indian nuclear warheads are the worlds least expensive nuclear warheads. When USA denied exporting wheat to India after our independence, a famous one liner of Lal Bahadur Shashtri "Jai Jawan, Jai Kisan" stirred up a green revolution producing a surplus amount of wheat in our country. No country will give you technology and that is why 'homi jahangir baba' started the atomic plants in India while Jawaharlal Nehru was begging western countries for the same.
If a country has to go forward and move farther than others then it should have better scientists, engineers with it than any other countries. Some foreign countries have realized it and they are now making sure that these talents don't stay in India but they rather serve their country and their companies. Our politicians have made sure that the public education system collapses where teachers don't come to school at all but instead they take private tuition to make more money. Schools have been told not to fail the students below 8th standard where as this is the age they will learn the fundamentals. Every year the standard of education is going down and there are more graduates with less knowledge. They are the one who will join public services, many through reservation but not based on talent. This will cause the deterioration of the society. Whom does it benefit ? the enemies of the country. There are many public schools which are being shut down and government makes sure that they delay the hiring of staff to government schools as much as possible so that the private schools which are run by missionaries along with the politicians as sitting partners or beneficiaries get the inflow of students. These private institutions only provide education based on how deep your pockets are which leads to a situation where only the rich get the education and it is a known fact that a rich kid has lesser reasons to study and plan his future based on his knowledge than a poor kid who cannot afford education. This leads to a society where slowly the collective intelligence deteriorates from generation to generation. Majority of the missionary run schools propagate an alien anti Indian culture and many of them are involved in an active, passive, direct or indirect conversion which leads to a situation where our own generation will act as agents of a foreign hostile culture.