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SB 3.22.3

tat-trāṇāyāsṛjac cāsmān
 doḥ-sahasrāt sahasra-pāt
hṛdayaṁ tasya hi brahma
 kṣatram aṅgaṁ pracakṣate
Synonyms: 
tat-trāṇāya — for the protection of the brāhmaṇas; asṛjat — created; ca — and; asmān — us (kṣatriyas); doḥ-sahasrāt — from His thousand arms; sahasra-pāt — the thousand-legged Supreme Being (the universal form); hṛdayam — heart; tasya — His; hi — for; brahma — brāhmaṇas; kṣatram — the kṣatriyas; aṅgam — arms; pracakṣate — are spoken of.
Translation: 
For the protection of the brāhmaṇas, the thousand-legged Supreme Being created us, the kṣatriyas, from His thousand arms. Hence the brāhmaṇas are said to be His heart and the kṣatriyas His arms.
Purport: 

Kṣatriyas are specifically meant to maintain the brāhmaṇas because if the brāhmaṇas are protected, then the head of civilization is protected. Brāhmaṇas are supposed to be the head of the social body; if the head is clear and has not gone mad, then everything is in proper position. The Lord is described thus: namo brahmaṇya-devāya go-brāhmaṇa-hitāya ca. The purport of this prayer is that the Lord specifically protects the brāhmaṇas and the cows, and then He protects all other members of society (jagad-dhitāya). It is His will that universal welfare work depends on the protection of cows and brāhmaṇas; thus brahminical culture and cow protection are the basic principles for human civilization. Kṣatriyas are especially meant to protect the brāhmaṇas, as is the supreme will of the Lord: go-brāhmaṇa-hitāya ca. As the heart is a very important part within the body, so the brāhmaṇas are also the important element in human society. The kṣatriyas are more like the whole body; even though the whole body is bigger than the heart, the heart is more important.