Leave somebody/something to the mercy/mercies of somebody/something
Definition 1 of 2
Pro Tip 1/3Negative Outcome
The phrase often suggests a bad result for the abandoned party. 
He left his plants to the mercy of a scorching sun.
Pro Tip 2/3Lack of Control
Implies a situation where no human control or influence can be exerted. 
They left the old cottage to the mercy of the relentless winds.
Pro Tip 3/3Emotive Imagery
The phrase can evoke strong feelings of vulnerability or abandonment. 
The sailors left the sinking ship to the mercies of an unforgiving sea.