If China, Pakistan and Sri Lanka attack India at the same time, how long will it take them to defeat India?

#First Scenario

The result will be a stalemate. It is purely hypothetical analysis to show interconnected everything is and nothing against any country. Here is why:
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First, let us take the low hanging fruit: Sri Lanka. Lankans received massive help from India to prevail over LTTE and end the civil war. The help included military (logistics, patrolling, arms), diplomatic (banning LTTE in Western nations) and political (prevailing over Tamilnadu's political parties). Indian central government is the only that can prevail over certain jigoist Tamil groups. Thus, if SL distracts Indian central government in any way, Tamil nationalist groups could send SL into a massive chaos and civil war. Thus, no way SL would mess with India.

Oil Throttle: India has a very strategic position in the global oil flow. East Asia's oil pass through India. At the start of any major war, India could throttle China's oil supplies and merchandise exports. India could also use its ally Iran at the head of Hormuz and South East Asian allies around Malacca to further the blockade.

Involvement of other powers: China's rise is not in any major power's interest. From US, UK, Germany to Russia, Japan and Korea are all scared of China's rise. For other Asian nations from Vietnam to Malaysia, a Chinese hegemony is a scary economic prospect. Although no one will directly involve, everyone will covertly provide aid to India.

Internal destabilization: Right now India has not involved in China's autonomous regions: Inner Mongolia, Xinjiang and Tibet. But, if push comes to the shove, India has the powers to foment trouble given a huge domestic Muslim and Buddhist population. India could also use its traditional links with Mongolia, Taiwan, Vietnam, Japan to encircle China in a covert war. Chinese society is precariously balanced especially when it comes to the non-Han minority and given all the economic tensions in the fast growing nation, it is easy to set the cat among the pigeons. In the same way, China could also do it in India, although with a little less success. 

Nuclear factor: While Pakistan has less to worry about India's nukes (nothing much to lose), China has everything to lose in a nuclear war with India. A cornered nation with nuclear power is the worst enemy you could have.

Thus, a Pakistan-China axis will never really work against India. The resulting mutual destruction is what preventing such an alliance from already happening.

#Second Scenario

While this may me a hypothetical scenario, it figures quite prominently in the Indian military's war scenario. Past news suggest that the Indian military has always accounted for a 2-front war with China and Pakistan. Let’s keep Sri Lanka out of this since its military is not large enough to pose a serious threat (No offence to the Sri Lankans, but it took them 30 years to get rid of the LTTE, an objective that was almost achieved in 3 years by 4 divisions of the Indian army. Contrary to popular perceptions (as presented here) the reality of a Sino-Pak-Indo conflict are quite different.

First I differ with one poster who compares the Chinese Military (CM) as a world-class boxer, India as street cop and Pakistan as a brother to the street cop. If there is one 1000 lb Gorilla (or the boxer) in the room of world's militaries it is the US military. The rest, including China, are nothing but street cops with varying degree of weapons and strength. To reason why, one only has to see that each US Navy CVN (Nuclear carrier) is the 20th largest air force in the world, having some of the most sophisticated aircraft , and there are 11 of them. And we are not even talking about the USAF.

Now let’s look at China, Pakistan and India. Between China and India there is this huge great wall, called the Himalayas, which makes PLA's 4000+ tanks pretty much useless. Infact in altitudes of 14,000+ feet which is where most of the war will be fought, its 100+ attack helicopters won’t be of much use either. The PLA will of course have a 1:3 advantage in artillery and with a nice logistics line running right up to the border they can get the big guns in numbers and fast. So the Himalayan war will be restricted to infantry and artillery where PLA has a 1:3 advantage. However, this advantage is dented because for being an overwhelming conventional force, a superiority of 1:6 is desired, more so in mountain warfare. Discounting support from the Air force (which we will dig into later) PLA will hard pressed for sustaining offensive operations in India. In 1962, when Indian military was the weakest, The PLA could not extend beyond Tawang in eastern India as it stretched their logistics line too thin. Even though PLA is better now, the Indian army has improved much more than 1962, and sustaining deep operations in the Indian theatre is not a tactical proposition for the PLA anymore.

In the plain desert of western India Pakistan's 1900+ tanks will be met by India's 3000+ tanks and its .5 million troop army by units of the Indian army backed by the para military BSF (with estimated 300K troops meant for the Pakistan border only). Assuming half of India's 1.2 million army is fighting China that still gives India a numerical advantage. On top of that the Indian army's Orbat is structured around 3 strike corps, (1,2 and 21 Corps) units of Independent army with heavy armor whose only goal is strike deep inside Pakistan. Infact the sole reason for existence of 1 Corps is to divide Pakistan in two, by deep push across the plains of Sindh. Rest of the so called Holding corps possess enough firepower to offensive operations. With a joint attack by Pakistan and China these powerful formations will be in defensive mode and it is highly unlikely that Pakistan will be able to undertake any offensive operations in Indian territory.

Regarding the Navy, though China may have a decent blue water capability it does not have the numbers or the experience to sustain operations in the backyard of the Indian Navy, the 2nd largest Navy in Asia. Indian Navy's 2 Carrier Battle Groups will pose a serious offensive threat to the surface fleet of both the Pakistan and Chinese navies. A naval blockade of India is unthinkable since this will bring the entire surface fleet of the PLAN and PN within strike range of shore based bombers and strike aircraft of the Indian Navy and air force. Unlike the US Navy or even the Royal Navy, PLAN lacks the air support firepower needed to protect its fleet from offensive ground based air operations.

That brings us to the role of AF's where PLAAF has a distinct advantage at least on paper. With 3000+ combat aircraft supported by mid-air refuellers the PLAAF is a force to content with. However apart from 250+ SU30MKK’s, 200+ Su-27s and 200+ JF-17’s rest of these aircraft are vintage Chinese variants of MiG 19 and MiG 21s. In the context of Sino-Indian conflict China has primarily 6 airbases to launch strike aircraft from. However, all of these have altitudes in excess of 4000 ft, which means the strike aircraft taking off will be carrying lesser payload. A PLAAF Su-30MKK for example will take off with 6 AA missiles rather than 8. All IAF airbases bordering China primarily those in Bareilly, Tezpur, Hindon and Bagdodra all at near sea level allowing Indian Su30MKI to takeoff at full payload. IAF has reserved most of the 150+ modern Su30MKI ( a more lethal aircraft than Su30MKK due to superior Radar and electronics) for the Sino border, while 300+ modernized MiG 21 , MiG 27’s,MiG 29 and Jaguar’s can take care of the PAF. In addition the IAF has 50+ Mirage 2000 fighter aircraft which have superb ground combat and electronic capabilities and these may be used for the Sino border too. On top of this the IAF has a force multiplier than neither PAF or PLAAF have, i.e. AWACS. Apart from Israel, IAF is the only asian airforce with Phalcon AWACS mounted in IL-76 platforms. This gives IAF the capability to look 1500 kms deep inside Chinese and Pakistan territories while flying in safe Indian air space. IAF will be aware of every PLAAF and PAF as soon as it takes off, given sufficient time for action. This is a serious issue for both PAF and PLAAF, and taking down the Phalcon’s should figure top in their air battle strategy.

All three countries have ballistic missiles and each one can hit every corner of each other with nuclear weapons. So this will probably be the last resort. However, here too the Indian’s posses a potential lethal edge that not many are aware off. The Indian research arm for ballistic missiles has consistently demonstrated the accuracy of its anti-ballistic missile system, where in 3 trials it has demonstrated critical success. What is even more remarkable that inbound missiles were also hit in exo-atmosphere, a capability known only with the US. Of course what helps the Indian is the Green Pine radar that US and Israel shipped to India in a low key event. If the anti ballistic missile system does work, rendering Chinese and Pakistani ballistic missiles useless it gives India an edge that the attacking neighboring can’t match. Note PLA only has a demonstrated capability to blow satellites which was widely publicized. However, satellites have a known trajectory and it is relatively very simple to blow them up without using sophisticated radars.

To conclude India vs China-Pakistan war will end in stalemate at best, and won’t give any tactical or strategic advantage to any nation. So much so that it’s motivation itself will be in doubt.

Source : QUORA