How knowledge of weapons (shastra) शस्त्र got lost?


Q: How did the knowledge of great weapons like Bramhastra got lost in Hinduism tradition?

Knowledge of great weapons like Brahmastra didn't get lost, they are still existing.

Simple proof is that Parashurama has the knowledge of Brahmasatra and he is a chiranjeevi[1],[2] i.e, he is alive now.

Narada said, 'That tiger of Bhrigu's race (viz., Rama), was well-pleased with the might of Karna's arms, his affection (for him), his self-restraint, and the services he did unto his preceptor. Observant of ascetic penances, Rama cheerfully communicated, with due forms, unto his penance-observing disciple, everything about the Brahma weapon with the mantras for withdrawing it. Having acquired a knowledge of that weapon, Karna began to pass his days happily in Bhrigu's retreat, and endued with wonderful prowess, he devoted himself with great ardour to the science of weapons. One day Rama of great intelligence, while roving with Karna in the vicinity or his retreat, felt very weak in consequence of the fasts he had undergone.

[Section 3, Rajadharmanusasana Parva, Santi Parva, The Mahabharata]

Q: As can be seen in Ramayan and Mahabharat, Gurus used to give knowledge of weapons to their shishya which were then used to protect Dharma. But eventually this knowledge was lost and Bharat couldn't fight invaders. How did this happen?

In Ramayan and Mahabharat, Gurus didn't give the knowledge of every weapon to every sishya. Inorder to give the knowledge of great weapons, honest guru atleast verify the eligibility of the sishya and sometimes the purpose also. If the sishya is either ineligible or asking weapon for bad purposes then the guru may not give the knowledge of weapon or may revoke it, if given earlier.

In Mahabharata, Drona did not give the knowledge of Brahmastra to Karna

Beholding that Dhananjaya was superior to every one in the science of weapons, Karna. one day approached Drona in private and said these words unto him, 'I desire to be acquainted with the Brahma weapon, with all its mantras and the power of withdrawing it, for I desire to fight Arjuna. Without doubt, the affection thou bearest to every one of thy pupils is equal to what thou bearest to thy own son. I pray that all the masters of the science of weapons may, through thy grace, regard me as one accomplished in weapons!' Thus addressed by him, Drona, from partiality for Phalguna, as also from his knowledge of the wickedness of Karna, said, 'None but a Brahmana, who has duly observed all vows, should be acquainted with the Brahma weapon, or a Kshatriya that has practised austere penances, and no other.' When Drona had answered thus, Karna, having worshipped him, obtained his leave, and proceeded without delay to Rama then residing on the Mahendra mountains.

[Section 2, Rajadharmanusasana Parva, Santi Parva, The Mahabharata]

Parasurama revokes the remembrance of Karna's Brahmasatras knowledge at crucial time

Unto the cheerless and trembling Karna, prostrated with joined hands upon earth, that foremost one of Bhrigu's race, smiling though filled with wrath, answered, 'Since thou hast, from avarice of weapons, behaved here with falsehood, therefore, O wretch, this Brahma weapon shalt not dwell in thy remembrance. Since thou art not a Brahmana, truly this Brahma weapon shall not, up to the time of thy death, dwell in thee when thou shalt be engaged with a warrior equal to thyself! Go hence, this is no place for a person of such false behaviour as thou! On earth, no Kshatriya will be thy equal in battle.' Thus addressed by Rama, Karna came away, having duty taken his leave. Arriving then before Duryodhana, he informed him, saying, 'I have mastered all weapons!'

[Section 3, Rajadharmanusasana Parva, Santi Parva, The Mahabharata]

As I told early, the purpose is also very important. Even the knowledge of weapons won't work if there is no purpose. The presence or absence of weapons solely depends on the purpose only. If there is no purpose then the weapons will be absent i.e., they won't work.

Arjuna's weapons are absent while he was trying to safeguard many people from attacking enemies

"Arjuna said, ‘....... Another incident has happened that is more painful than this, O thou that art possessed of wealth of penances. Repeatedly thinking of it, my heart is breaking. In my very sight, O Brahmana, thousands of Vrishni ladies were carried away by the Abhiras of the country of the five waters, who assailed us. Taking up my bow I found myself unequal to even string it. The might that had existed in my arms seemed to have disappeared on that occasion. O great ascetic, my weapons of diverse kinds failed to make their appearance. Soon, again, my shafts became exhausted. That person of immeasurable soul, of four arms, wielding the conch, the discus, and the mace, clad in yellow robes, dark of complexion, and possessing eyes resembling lotus-petals, is no longer seen by me. .........’

"Vyasa said, ‘......... All this has Time for its root. Time is, indeed, the seed of the universe, O Dhananjaya. It is Time, again, that withdraws everything at its pleasure. One becomes mighty, and, again, losing that might, becomes weak. One becomes a master and rules others, and, again, losing that position, becomes a servant for obeying the behests of others. Thy weapons, having achieved success, have gone away to the place they came from. They will, again, come into thy hands when the Time for their coming approaches. The time has come, O Bharata, for you all to attain to the highest goal. Even this is what I regard to be highly beneficial for you all, O chief of Bharata’s race."

[Section 8, Mausala Parva, The Mahabharata]

Thus possessing of weapons are useful only if time allows it. Else they are useless. So, one need to understand that weapons make their appearances for establishing dharma at proper times by deserved person only.

Q: Why weren't Devas invoked through yajnas when Sanatan Dharma followers were ethnically cleansed at the rate of millions by foreign invaders? At time of Mahabharat Indra himself helped Arjuna by granting him weapons. What happened now?

As you can see in Arjuna-Vyasa conversation that knowledge of weapons will not be present if some event is destined to happen. Although Arjuna possess knowledge of weapons, weapons did not make their appearance when thousands of Vrishni ladies were carried away by the Abhiras. This is because of the reason that the event need to be happened because of curse. The weapons made their appearances during Kurukshetra war, but not during the above mentioned attack.

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