Brahmasthira - The ultimate weapon

This powerful form of weapon is used by great warriors at many stages of war. This weapon is invoked by mantras where an object like bows or even a blade of grass can be transformed into a mighty Brahmasthira. Brahmasthra is of different varieties, an advanced version being Brahmashira. The energy of the Brahmasthira is based on the capacity of the launcher and the type of mantra he utters. For instance Karna uses Brahmasthira which was baffled by Arjuna belong to less powerful category.

Karna, invoking the brahmastra, "showered his shafts upon Dhananjaya, and once more made an effort to extricate his car. Partha also, by the aid of the brahmastra, poured arrowy downpours upon Karna".

While the one used by Ashwathama and baffled by Arjuna belongs to a highly powerful variety. Where the total atmosphere trembles and ved Vyas and sage Narada appear immediately for mediation.

We can understand the difference from the description itself. The mighty Pasuputa asthra given by Shiva to Arjuna also belongs to a variety of Brahamasthira.
" Arjuna said, 'O illustrious god having the bull for thy sign, if thou wilt grant me my desire, I ask of thee, O lord that fierce celestial weapon wielded by thee and called Brahmasira--that weapon of terrific prowess which destroyeth, at the end of the Yuga the entire universe--
Shiva replied, 'O powerful one. I will give to thee that favourite weapon of mine called the Pasuputa. O son of Pandu, thou art capable of holding, hurling, and withdrawing it. Neither the chief himself of the gods, nor Yama, nor the king of the Yakshas, nor Varuna, nor Vayu, knoweth it. How could men know anything of it? But, O son of Pritha, this weapon should not be hurled without adequate cause;"

Containing a brahmasthira

During the war of Mahabharatha weapons powerful enough  to modern day nuclear bomb were not only used but also neutralised. There were technology to rapidly neutralise the power of nuclear spread. The following is description on the encounter between Ashwathama and Arjuna. Ashwathama's uses brahmasthira by inspiring a blade of grass with proper mantras and converts it into that powerful celestial weapon. A fire then was born in that blade of grass, which seemed capable of consuming the worlds.

Arjuna immediately uses a weapon to neutralise it blazed up with fierce flames like the all-destroying fire that appears at the end of the yuga (world). Similarly, the weapon that had been shot by Ashwathama blazed up with terrible flames within a huge sphere of fire. Numerous peals of thunder were heard; thousands of meteors fell; and all living creatures became inspired with great dread. The whole world seemed to be filled with noise and assumed a terrible aspect with those flames of fire. The whole earth with her mountains and waters and trees, trembled.

In the words of rishi (enlightened Saint) Vyasa,
"The region where the weapon called brahmashira is baffled by another high weapon suffers a drought for twelve years, for the clouds do not pour a drop of water there for this period".

We know how Japan witnessed (last century) this in Hiroshima and Nagasaki