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