Who would win if the USA alone invades india ?

Even if USA does invade India, I have serious doubts it will be able to conquer it, more so win it. If you need proof take into account the military conquests of Alexander, Mahmud Ghanzni, the Mongols, the Huns,   Of course Pakistan would provide its bases and armed forces units as forward units. And much like Italy during the World War II with its imprisoned Mafia dons in Sicily, the USA will strike a deal with the the various main and fringe terrorist organisations to provide the auxilliary shock troops.

The Indian army has at all times 4,00,000 standing troops in reserve. This count excludes the military, navy and air force deployments at present and the initial troop count. We have a gentleman named Ajit Doval, who is the current NSA. You may have read about the military operation in Myanmar ( I apologise to our friends in Myanmar for the inconvenience caused, but we cannot let anyone just kill our soldiers and not pay for it ). That was done under the supervision of Mr. Doval and completely backed by the the Cabinet of the Union of India.We still have a very formidable intelligence unit. It's called R&AW. Then we have our navy and Air Force. For Air Force, in case any one has forgotten please follow upon 2004 Cope Exercise. For our naval capabilities, we sank PNS Ghazi, and in case US Navy makes it past Andamna and Nicobar, or the Lakswhadeep, we will be sure to send the debris if we want to find. Also read about the recently inducted special ops branches of the services. They are asked to kill first, question later. And do not forget the Paras. They had a lad named Capt. Vikram Batra. Ask the veterans of Tiger Hill who he was. Now we have equally talented men. I say men, not man.

Then we our own set of tricky little blighters, as the British say. They are called Maoists. These guys have created our own personal little Vietnam for us. They are trained in jungle warfare, camouflage and despite the lack of resource can mingle effectively with the local populace. Given the current anti-American sentiment, I really do not think the guys from ISIS and other terrorist organisation will make the whole conquer India easily. Then you have us, the crazy little Indians, who are next to useless, but given the temptation to fall on to mob frenzy we can attempt to wreck havoc. We are extremely patriotic, and crazy ( the waiting in traffic will give you a better idea).

Here comes the fun part. Hypothetically, our armed forces are defeated and any other possible threat ( read Maoists), Russia will not be an idiot just to watch by the sidelines. Given the fact that it neutralised the US fleet sent to aid Pakistan during the 1971 war, it can also target major locations in the USA, both East and West coast. China, which despite having a love-hate relationship with India, can virtually destroy USA if it starts dumping the debt taken by them on the global market, simply to avoid an overlord in the South Asian subcontinent. It is relatively stronger both financially and militarily. Then there is Israel. I have mad respect for these guys. I really hope, when such a situation arises, we can count on their support. So, in order to invade India, USA will have to simultaneously attack Russia, China, Israel, Afghanistan. Oh, and I also forgot, the NATO.