The Kushana Empire was one of the most powerful Empires in ancient India. Kushanas were turbulent tribes, who established their Empire in ancient India after confronting several obstacles. The Kushanas were mighty conquerors and the Kushana Empire stretched to a vast extent including the entire parts of Northern and northwestern India. Kushana Empire not only extended within the limits of India, but also to the countries of Central Asia. A chain of vibrant rulers belonging to the Kushana dynasty ruled ancient India and during their time India attained a thriving progress in the fields of art, architecture, literature and sciences. However the Kushana Empire was destined to decline due to the weak successors of Kanishka or the later Kushanas. Historians have opined that Vasudeva was the last in chain of the important Kushana kings. After the death of Vasudeva, forces of disintegration set within the Kushana Empire, which was already in the verge of downfall. Though after the death of Vasudeva history of the Kushana Empire was shrouded in mystery, yet some facts about the causes of downfall of the Kushana Empire can be discerned from the available epigraphic and numismatics evidences.

Historians have opined that after the demise of Vasudeva I, the Kushana throne was ascended by Vasudeva II and later by Kanishka III. But there is nothing to prove the authenticity of this fact. Moreover historians contradict over their views regarding the relationship between the kings. Hence it is generally believed that whoever the successor of Vasudeva was, ruled for 30 years and during this time Kushana Empire completely declined.

Forces of disintegration had started showing signs within the Kushana Empire, during the time of the later Kushana Kings. Kushana Empire was a vast kingdom and to maintain the local and provincial administration, the antecedent of the Kushana Dynasty appointed local chiefs. But they themselves kept a watchful eye towards the proper maintenance. But the later Kushanas were incompetent enough and they could not hold their authority properly over the local and provincial administration. Hence during their time, the local feudatories and the chiefs raised the standard of revolt and assumed independence. The later Kushanas were not even mighty militants like their predecessors and hence could not suppress the rebellion initiated by the local chiefs. The Saka Satraps of Western and Central India who offered their allegiance to the Kushana suzerain became independent taking advantage of the weakness of the later Kushanas. In the meantime the Nagas rose to power in this region, defying the Kushana authority. They celebrated their victory by performing a horse sacrifice. The Puranas mention that the Mathura and Padmavati regions were ruled by the Nagas before the Guptas. It has also been mentioned in the Puranas that the Nagas suppressed the Kushana dominance in the areas of Mathura and emerged victorious. The Yaudheyas ousted the Kushana supremacy in wide regions of the Sutlej valley in Punjab. To the east of Mathura, the Maghas also rose to power. The Kunidas overthrew the weak Kushanas from the regions of Yamuna and Sutlej. The regions of West Punjab and northwest was occupied by the tribes called Shakas and Shiladas. Some other tribes called Sanchi, Jhansi and Bhilsa occupied the other fragmented portions of the Kushana Empire. Internal dissension within the Kushana Empire weakened the very foundation of the Kushana Empire in India, which ultimately led to the downfall of the Kushana realm.

When the very foundation of the Kushana Empire started to collapse within the heart of India, the successors of the Kushana King Vasudeva I precariously clung to their position in the Northwest Frontier Province and Afghanistan. But this last stronghold of the Kushanas were suppressed and shattered completely by the Sassanian invasion from Persia. Smith suggested that "Persianising of the Kushana coinage of Northern India" in the third century A.D indicates the decay of the Kushana power due to Persian invasion. The Sassanian power of Persia founded by Ardeshir I was extended by his successors to the limits of Bactria and Northwestern India. The Kushana chiefs who reigned in this region could not put up a formidable protest against the terrifying Sassanian forces. Hence they had to bow down to the Sassanian conquerors and became their vassals. Te Naksh-I- Rustam inscription of 262 A.D states that Kashgarh, Khotan, Yarkhand was under the suzerainty of the Persian king Sapur I. It means that the entire Central Asian province was lost to the Kushanas. According to Dr. B.N. Mukherjee, the Indian tribes like Yaudheyas, Arjunayanas and others were not responsible for the breakdown of the Kushana Empire, rather the Sassanian invasion led by Ardeshir and his successors installed forces of disintegration within the Kushana Empire. Mukherjee has also added that the downfall of the Kushanas was accentuated by the Indian tribes.

However the vigour of the Kushanas after their complete disintegration was not entirely exhausted. Some Kushana tribes called the Kidara Kushanas flourished in Kabul Valley. In 5th century A.D. they fiercely resisted the Hun invaders. But the rest of the Kushanas in Northwestern India were intermingled with other races. But they retained their identity as Kushana descendants for long. Till the time of Samudragupta, the Kushanas did not lose their respective identity, though the former compelled them to offer him allegiance and forfeited their own distinctiveness. During the time of Samudragupta the Kushana descendants still existed, but were reduced as petty locals and provincial chiefs under the sovereignty of Gupta kings. These petty local chiefs, who belonged to the Kidara Kushana descendant, according to Chinese annals originally belonged to the Yue-Chi tribes of the Kushanas. However the theory is doubtful enough because there is no such evidence to support the Chinese annals. As a whole the remnants of the main Kushana Empire in the border regions had completely vanished under the Sassanian invasion in or about 3rd century A.D.