"...when a system at equilibrium is subject to an external stress,"...when a system at equilibrium is subject to an external stress,"the equilibrium will move so as to oppose the stress....."the equilibrium will move so as to oppose the stress.....
Explanation:
It is important to realize that "oppose"oppose does not equal "counteract"counteract. Under the new conditions, a new equilibrium may develope. We speak of the initial response of a system whose equilibrium is perturbed.