SB 3.6.31

bāhubhyo ’vartata kṣatraṁ
 kṣatriyas tad anuvrataḥ
yo jātas trāyate varṇān
 pauruṣaḥ kaṇṭaka-kṣatāt
Word for word: 
bāhubhyaḥ — from the arms; avartata — generated; kṣatram — the power of protection; kṣatriyaḥ — in relation to the power of protection; tat — that; anuvrataḥ — followers; yaḥ — one who; jātaḥ — so becomes; trāyate — delivers; varṇān — the other occupations; pauruṣaḥ — representative of the Personality of Godhead; kaṇṭaka — of disturbing elements like thieves and debauchees; kṣatāt — from the mischief.
Translation: 
Thereafter the power of protection was generated from the arms of the gigantic virāṭ form, and in relation to such power the kṣatriyas also came into existence by following the kṣatriya principle of protecting society from the disturbance of thieves and miscreants.
Purport: 

As the brāhmaṇas are recognized by their particular qualification of inclination towards the transcendental knowledge of Vedic wisdom, so also the kṣatriyas are recognized by the power to protect society from the disturbing elements of thieves and miscreants. The word anuvrataḥ is significant. A person who follows the kṣatriya principles by protecting society from thieves and miscreants is called a kṣatriya, not the one who is simply born a kṣatriya. The conception of the caste system is always based on quality and not on the qualification of birth. Birth is an extraneous consideration; it is not the main feature of the orders and divisions. In Bhagavad-gītā (18.41-44) the qualifications of the brāhmaṇas, kṣatriyas, vaiśyas and śūdras are specifically mentioned, and it is understood that all such qualifications are needed before one can be designated as belonging to a particular group.

Lord Viṣṇu is always mentioned as the puruṣa in all Vedic scriptures. Sometimes the living entities are also mentioned as puruṣas, although they are essentially puruṣa-śakti (parā śakti or parā prakṛti), the superior energy of the puruṣa. Illusioned by the external potency of the puruṣa (the Lord), the living entities falsely think of themselves as the puruṣa although they actually have no qualifications. The Lord has the power to protect. Of the three deities Brahmā, Viṣṇu and Maheśvara, the first has the power to create, the second has the power to protect, and the third has the power to destroy. The word puruṣa is significant in this verse because the kṣatriyas are expected to represent the puruṣa Lord in giving protection to the prajās, or all those who are born in the land and water. Protection is therefore meant for both man and the animals. In modern society the prajās are not protected from the hands of thieves and miscreants. The modern democratic state, which has no kṣatriyas, is a government of the vaiśyas and śūdras, and not of brāhmaṇas and kṣatriyas as formerly. Mahārāja Yudhiṣṭhira and his grandson, Mahārāja Parīkṣit, were typical kṣatriya kings, for they gave protection to all men and animals. When the personification of Kali attempted to kill a cow, Mahārāja Parīkṣit at once prepared himself to kill the miscreant, and the personification of Kali was banished from his kingdom. That is the sign of puruṣa, or the representative of Lord Viṣṇu. According to Vedic civilization, a qualified kṣatriya monarch is given the respect of the Lord because he represents the Lord by giving protection to the prajās. Modern elected presidents cannot even give protection from theft cases, and therefore one has to take protection from an insurance company. The problems of modern human society are due to the lack of qualified brāhmaṇas and kṣatriyas and the overinfluence of the vaiśyas and śūdras by so-called general franchise.