Who are 'us' here, BTW.
Countries with highest number of guns in civilian hands - where does India rank?
India has more number of guns in private hands than Pakistan.
And Indian police force is quite substantial : 2.1 million + 300K CRPF. India will be fine. Of course mosquitos without food and water and in enemy territory is another matter.
And lets not even talk about NSGs. If they are deployed, it will be a slaughter. They are experts of urban warfare.
Meanwhile, your only point of entry can be Punjab. Rann of Kutch and Rajasthan deserts are not survivable without enough preparation. So... roughly India known where you are coming from, where you will be and well, in '71 90K regular forces surrendered and in 20XX, 5-7 million will surrender.
If anything, India will face a massive refugee problem from Pakistan, not exactly an armed civil rag-tag militia.