If the full scale war between China and India break out, this idea of what the world will form an alliance to against China is imaginary. China has probably more international friends than India does. Our borders with other neighboring countries are peaceful. You fellow Indian posters keep saying that China is having border disputes with all of its neighbors. That is not true. Since 1980s, China settled most of land border disputes with neighboring countries except those with India and Bhutan.
The image of what China is a bully in Asia and was hated by all of its neighbors was painted for years by your media and your government. That is a pure propaganda which is understandable. But if you make decision based on it, you just bring a disaster to your home. Unfortunately, I found that most Indian indulge in those imaginary worlds.
Even though this propaganda is true, you cannot count on those neighboring countries to join your war. Most likely they will keep their mouth shut, not even a moral support to you. Why? Let me tell you why: because they are too small. For example, Mongolia’s population is 2 million and Inner Mongolia has 20 million and 90% of them are Han Chinese. The safest way for them is to watch which country survives this war. Why do they want to bring the fire to themselves? Japan probably will give you some lip service, but no military support.