What is the ruling on one who breaks a covenant with Allah, such as vowing to Allah not to smoke and then smoking?

If the covenant is an oath, then violating it necessitates an expiation for the oath. However, if it is a covenant devoid of an oath, then breaking it with Allah, especially concerning the abandonment of disobedience, is more grievous, but no expiation is required. Rather, repentance, seeking forgiveness, and resolving to abandon the disobedience suffice.