Because if your allies get attacked, you come for help. That's what being allies means. If US doesn't participate in history's biggest war, they'll cease to be a world power. And US will never side with China.

India isn't an ally of the US as far as I know, it's a leader of the non aligned nations in the UN and accused of sanctions busting for cheap oil from Russia. It's not a member of any of the NATO class treaty bodies, and it actively purchases arms from the FSU, and repudiated the non proliferation treaty.

True, India is an ally of Russia and not USA/West. But in case of war, USA can do three things:

1. Support China: China has similar military to USA and 4x the population. If USA does support them, china will strongarm literally every country in the world and become the hegemon.

2. Stay Neutral: This would mean every bigger country can attack and capture nearby smaller ones. The reason expansion wars don't happen now is because of the fear of US.

3. Support India: If US is to counter China, this means supporting India.

4. Say the UN won't let us get involved and exploit it for political gain.

What the US almost always does. But I am sure we will send some troops to sit on the sidelines and make sure it is a fair war.

>This would mean every bigger country can attack and capture nearby smaller ones.

Which one is "the smaller one" from this pair?

That's not quite how things work. You may call for your allies to help, but they may decide that they don't want to help you out, or only agree to help you out in indirect ways.

If that were true than every war would develop into a world war.