The EU, USA, and Russia would probably stay out of a conflict. They may publicly condemn an attacks but they are all too reliant on Chinese goods to do anything. The only possibility of physical intervention is if there are rampant human rights violations.
The UN won't be able to do anything either since China is a permanent member of the Security Council.
However, a Chinese attack on India would never happen. The casualties on either side would be devastating and if worst comes to worst, India would threaten with nuclear weapons.
The EU, USA and Russia will only support India. These three have their own reasons for disliking the Chinese.
No country would support actively India . India had seen durin gBangla crisis
Indian army may not hold on its own for long without resorting nuclear escalation
china will **** up india. China is the best. India sucks & its fucked up.
It is wrong to expect support. We must learn to defend ourselves.
If it is happen then which country get more support..european union,usa and russia which country they will support.