There are many flawed premises in your question.
Comparing Apples with Oranges
First mistake you have made is, comparing or equating very different entities.
Pakistan is not Tibet and India is not China. Moreover, 2016 isn't 1949.
Pakistan has a pretty strong military (Ranked 13th in the whole world by GlobalFirePower) which is of course still not as strong as Indian military but they ought to able to defend themselves. Please do not cite war of 1971 here because the circumstances aren't what forced a Pakistani defeat in 1971. In 1971, Pakistan lost because:
- Local population was against them
- Their armed forces were isolated from Pakistani heartlands by a 1000 Miles of Indian territory lying between West Pakistan and Bangladesh.
- Pakistan did not have a strong navy to protect supply routes to Garrisons in Bangladesh.
- Pakistan had to fight with Indian backed rebels in the rear while fending off regular Indian troops on the front.
- Pakistan had virtually no airpower in the east, capable of providing air support and defense to the ground troops.
If let's assume, India is to invade Pakistan now, do you see any of these decisive factors playing in India's favor? If these things do not happen, we see results of other Indo-Pak wars:
- Kashmir War 1948 - Stalemate, Partition of Kashmir in Pakistani and Indian zones
- War of 1965 - Stalemate
So if those two wars are any indicator, under normal circumstances, India can't "conquer" Pakistan.
Not to mention, Pakistan has nukes and so does India. So neither can afford the adventure of a war without decimating each other completely. There will be naught for the conquest for either of you, except nuclear waste.
So as Evident, Pakistan will be a far tougher foe for India than Tibet was for China.
Material Capability to fight a war of Conquest
However, neither country is capable of fighting a decisive war. They can only fight wars for one to three weeks, after that their supplies start dwindling.
Things become more complicated as both are dependent on foreign suppliers for their arms and International community has always banned sales of arms to both nations in the past in event of a war. So it is safe to say, neither can fight a war of conquest and will be forced to negotiate a ceasefire within few weeks of fighting due to material and logistical constraints.
Historically "Indian" Lands
I find it quite interesting that while you claim that the lands have been Indian for centuries, you refrained from pointing out that so have been the people.
Creation of Pakistan is not quite as simple as you have described here however. Pakistan did not just came to existence out of the blue. The people had been there for centuries and so had been the sense of being a distinct group, different from the rest of India.
Indians have been a multi-faith people for thousands of years. The problem however was that given the Muslim rule on India, the faith of Muslims was seen more like ethnicity than as a religion. Despite living together, Hindus and Muslims of India didn't quite become one nation at all.
If you look at historical context, you will find that heroes and villains of the two people are quite opposite.
- In Umayyad invasion of India, Muslims tend to consider Muhammad bin Qasim their hero while Hindus consider Raja Dahir as their hero.
- In Battle of Panipat, Muslims consider Ahmad Shah Abdali their hero while Hindus consider Marahtas their hero.
The list is too long, I will just cite these two only. As you can see, we see the two pitted against each other in battle. Of course, Then the British came along and some sense of one nation started getting developed however that wasn't enough to satisfy the Muslim majority regions who demanded to be separated from Independent India as they did not want to live under a Hindu majority rule, which obviously a democratic India would have been given that overwhelming majority was Hindu.
TLDR, Indian North-Western and Frontier-Eastern regions breakaway and Indian people in those regions adopt a new national identity, Pakistan.
The historical claim absurdity
Of course there was no Pakistan before 1947, but was there a Republic of India before 1947?
Since 1200s, there had always been a central Muslim state at head of India which was seen as theirs by the Muslims and evidently as occupiers by the Hindus as evident by the many revolts against those Muslim rulers.
Republic of India was founded on 15th of August 1947 and therefore has no historical claims on Lands to the West.
When you say India has historical claims, you are implying to be successors of Pre-British Indian states. Which states were those? Mughal Empire? Muslim. Lodhi Sultanate before them? Muslim. Sayyid Dynasty before them? Muslim. Tuhglaq dynasty before them? Muslim. Khalji Sultanate before them? Muslim. The slave dynasty before them? Also Muslim. So in that way, Pakistan could also lay claim to be successor of those Muslim Empires and assert that they have a historical claim on India.
Should Iran have a claim on India based on Persian Empire's pre and post Islam conquests of Indian lands? Should Afghanistan have a claim on India based on Afghan conquests of India?
Of course the "Historical claim" bit is amusing, It is just one of the many pretexts a powerful country can use against a weaker neighbor. If we talk about Historical claims, everyone would have one against everyone.
It's not that hard to speculate about International reactions.
The world as a rule has evolved beyond "conquest", "reconquest" or "annexation". Countries go to war, Countries get occupied but they aren't annexed now.
Of course there is the recent annexation of Crimea by Russia. Countries have condemned it, most don't recognize it however there haven't been any serious efforts to liberate the said region.
Then there is annexation of Kuwait by Iraq, based on Historical claims. The result was the First Gulf war which resulted in near-destruction of Iraq and liberation of Kuwait from Iraqi jaws.
So as you can observe, In case of Stronger Russia, nothing more than sanctions or condemnations was done. In case of Weaker Iraq, World started a war to undo the annexation.
India isn't Russia but it isn't Iraq either that any misadventure might result in global war against them. However Indians can expect that key global and regional power players would be very much against any such act.
Chief among them would be:
- USA - Has a vested interest in Pakistani state as a tool to further their agenda in Central and South Asia.
- China - Has a vested interest in Pakistan as a key ally against India.
- KSA - Has a vested interest in Pakistan and uses them to further their agenda.
- Iran - Relations not as they were back in the days of the Shah but still doesn't appear to be welcoming to any Indian aggression.
- UK - Has an interest in Pakistan because Pakistan is a member of the Commonwealth and British Troops are in Afghanistan.
- EU & NATO - Have an interest in Pakistan because of Afghan war.
- Russia - Traditionally not friends but getting closer with Pakistan and in search of new partners.
- Pakistan - I have a feeling that the nuclear armed state would have a word or two to say about their proposed annexation by India. I suspect them to be quite strong words.