Whenever and object is able to change its state of rest or state of uniform motion, it being applied by one of the unbalanced forces.
Now in this sentence why is it said state of uniform motion and not simply motion?
If the body were in non-uniform motion that it would be under the influence of external unbalanced forces to begin with and would be either accelerated or decelerated as it moves along. This is why we use the term uniform motion and any force affecting it would be unbalanced in nature.