Is Brahmastra a nuclear weapon?

The answer is no.

Brahmastra is a weapon based on the power of the Gayatri mantra.

It is a divyastra.

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What is a divyastra?

Use of mantras in warfare used to be very common in the ancient history of India.

These weapons are called divyastras.

Dhanurveda is a Upaveda dealing with warcraft such as archery.

Dhanurveda talks about seven primary divyastras - Brahmastra, Brahmadandakam, Brahmashira, Pashupatastra, Vayavyastra, Agneyastra, and Narasimhastra.

Divyastras are all essentially mantras.

Mahadeva is the governing God of all weapons including divyastras.

How does someone obtain a divyastra?

The deeksha of these mantras is obtained from a guru who is already accomplished in that mantra.

Having received the deeksha, the mantra is to be chanted a large number of times.

This procedure is called purascharana.

Then only the mantra works.

The number of chantings required for Brahmastra to work is one nikharva, 1 billion times.

जप्त्वा पूर्वं निखर्वं चाभिमन्त्र्य विधिवच्छरम्

Then it can be used as a weapon.

How do you summon Brahmastra?

A simple arrow or even a blade of grass can be energized with the mantra by chanting a few times before being shot.

What is the mantra of Brahmastra?

दादिदान्तं च सावित्रीं विपरीतां जपेत्सुधीः

Gayatri mantra chanted in the reverse direction.

Brahmastra can uproot entire vamshas.

How is Brahmastra withdrawn?

यथाक्रमं तादिदान्तां जपेत्संहारसिद्धये

In order to withdraw Brahmastra, the Gayatri mantra is chanted in the normal way.