Brahma said that a Brahman should earn livelihood. The best livelihood for a Brahman is receiving without asking (Vritta). Second to this is collecting what has been abandoned, like grain scattered in fields, market places, or granaries. Third best is the livelihood received for completing Yajna, ritual sacrifice of service or materials - but this should always be accepted. Fourth best is received for educational activities, research, development, and other auspicious activities.
If these are not possible due to any number of circumstances, a Brahman may undertake the livelihood of a Kshartriya: the Brahman Kshartriya must master the vedas as well as the different weapons. Battle should always be the last option for the Brahman Kshartriya. But all Brahmans should adopt the Kshartriya livelihood if required for right-defense - or the defense of others, or to ensure successful yoga. Then should all Brahmans wage war, in mutual defense.
A Brahman may also undertake, due to several emergent and urgent circumstances, the livelihood of a Vaisya, through industry, especially in agriculture. But in this, the Brahman Vaisya must still perform their duties as a Brahman. Exploitation of customers and dishonesty are dangerous temptations to the Brahman undertaking Vaisya.
It is not by birth one becomes a Brahman, but by self-restraint and self-control. Anything contrary to these undoes a Brahman.