Unlike fire-based weapons of current era, in ancient India or Bharat (भारत), weapons were of many kinds. Weapons were called Astra ( अस्त्र ) in Samskrtam. As you may have seen in Ramayana (रामायण) and Mahabharata (महा भारत ), there were various types of astras made of fire (Agni), rain (Varuna) and wind (Vayu). In addition, there were astras made out of venom (Naga), intense light (Surya), lightening (Vajra) and mountains (Parvata). Typically, each of these astras was named after a Deity. As an example, fire based astras had Agni or god of fire as the Deity.
In addition to these Deity-based astras, trimurtis (Brahma, Vishnu and Shiva) had their own more powerful astras.
Lord Vishnu’s astra was Narayanaastra. It fires a powerful tirade of millions of deadly missiles simultaneously. The intensity of the shower increases with increase in resistance. The only way of defense towards this missile, was to show total submission before the missiles hit. This in turn will cause this astra to stop and spare the target. Ashwathama, a Kuru warrior in Mahabharata unleashes this astra on the Pandava forces. Lord Krishna, who was an avatar of Vishnu, tells Pandavas and their warriors to drop their astras and lie down on the ground, so that they all surrender completely to the power of the astra. This secret of nullifying the power of this astra by this method was known only to three warriors namely Drona, Aswathama, and Krishna. Even Arjuna was not aware of this secret. It was also said that this astra can be used only once in a war and if one tries to use it twice, then it would devour the user's own army.
Lord Shiva’s astra was Pashupatastra. It was discharged by the mind, the eyes, words, or a bow. Never to be used against lesser enemies or by lesser warriors, the Pashupatastra was capable of destroying creation and vanquishing all beings. On a side note, PashupatiNath is the most important of all Shiva temples located in Kathmandu, Nepal. This Pashupatastra was used in the Mahabharata war by Arjuna to kill Jayadratha, and by Meghanada on Lakshmana in Ramayana. It returned without causing any harm since it can be used only to uphold Dharma.
The deadliest of all astras was Lord Brahma’s Brahmastra, capable of killing even Devas. It was used by Ashwatthama on Parikshit. It was considered the deadliest astra (similar to the hydrogen bomb). When the Brahmastra was discharged, there was neither a counter attack nor a defense that could stop it. The Brahmastra never missed its mark and had to be used with very specific intent against an individual enemy or army, as the target faced complete annihilation. It could only be used once in a lifetime. The user would have to display immense amounts of mental concentration. According to ancient Sanskrit writings, the Brahmastra was invoked by a key phrase or invocation that was bestowed upon the user when given this astra. The astra was also believed to cause severe environmental damage. The land where the astra was used became barren and all life in and around that area ceased to exist, as both women and men became infertile.
In the Ramayana, passages describe how the Brahmastra was used by Lord Rama as the final blow against Ravana, during their battle in Lanka. Srimad Bhagavatam describes the effect of Brahmastra in the following sloka.
It means, "When the rays of the two Brahmastras combined, a great circle of fire, like the disc of the sun, covered all outer space and the whole firmament of planets."
In a discourse, Swami Prabhupada explained "The heat created by the flash of a Brahmastra resembled the fire exhibited in the sun globe at the time of cosmic annihilation. The radiation of atomic energy is very insignificant in comparison to the heat produced by a Brahmastra. The atomic bomb explosion can at utmost blow up one globe, but the heat produced by the Brahmastra can destroy the whole cosmic situation. The comparison is therefore made to the heat at the time of annihilation."
In summary, contrary to popular belief that Hinduism is a soft religion that promotes nothing but ahimsa, we find numerous and potent astras in our Puranas that were needed to protect Dharma. That is why we say Dharmo Rakshati Rakshitah, or Dharma protects those who protect Dharma.