From the numbers alone, there are 1350 Million people in India. 75Million in Korea (25 North, 50M south)
If it were a hostile takeover, Indians are outnumbering Koreans 18 to 1.
India, while having a far greater population, is very divided. Each state is very culturally different, even as far as the languages they speak. Also, they aren’t generally armed with guns, and any form of combat training is very minimal in most states of India, with the exception of northern India.
However, India has the 2nd largest military in terms of numbers. 1.4 Million active members, with nearly 2 million reserve units..... let alone the civilian population that they could militarize if need be.
North Korea, has about 1M active military personnel, and 5.5M reserve, with far superior training and equipment.
South Korea has about 500,000 active members and 3M reserve.
I know you said nuclear weapons are off the table, but Korea (supposedly) has a variety of chemical weapons and other forms of WMD. You didn’t mention if India was invading north or South Korea, so we have to assume it’s both.....
If North Korea and South Korea (technically making allies, which you said was off the table, but you said invading Korea so we have to assume they’re together for the common good at this point) .... if they joined forces as one Korea, I don’t think India could invade them from a purely militaristic standpoint..... but here’s where the tables turn.
India would have to take a more strategic course of action to be successful. I don’t see brute force working here.
Let’s say India didn’t “invade” in a traditional sense. They’re not sending untrained people to the border and lining up about to go in. Instead, they take the next 2 years and send over “tourists”. They’re getting their people across the border and infiltrating the country without being seen or noticed. By the time a couple of years pass, they have 10s of MILLIONS of their troops and militarized citizens over there, and barely touched their own population numbers. At this point they focus in on North Korea, and overthrowing its government, taking the head straight off of the beast. With North Korea overthrown, and not worrying about an uprising, because the citizens actually are welcoming a change, they then launch a full military attack on South Korea, which is a complete win for India.