After USA pulls out it's troops from Afghanistan, And If there's anarchy in Af again, Who would step in?

There is already fears that if the US presence leaves, Many Afghan people will lose jobs and seek insurgency. Afghanistan isn't that stable in economy. In case, there's a problem again, Which country will take steps towards it? China? Russia? India? Pakistan has so much internal problems, leave that. And How would the USA, and NATO react?