He slew the Dragon lying on the mountain: his heavenly bolt of thunder Tvashtri fashioned. Like lowing kine in rapid flow descending the waters glided downward to the ocean.
Impetuous as a bull, he chose the Soma and in three sacred beakers drank the juices. Maghavan [Indra] grasped the thunder for his weapon, and smote to death this firstborn of the dragons.
When, Indra, thou hadst slain the firstborn of dragons, and overcome the charms of the enchanters, then, giving life to Sun and Dawn and Heaven, thou foundest not one foe to stand against thee.
Indra with his own great and deadly thunder smote into pieces Vritra, his greatest enemy. As trunks of trees, when axe hath felled them, lie low on the earth, so lies the prostrate dragon.
He, like a mad weak warrior, challenged Indra, the great impetuous many-slaying Hero. He, brooking not the clashing of the weapons, crushed -- Indra's foe -- the shattered forts in falling.
Footless and handless still he challenged Indra, who smote him with his bolt between the shoulders. Emasculate yet claiming manly vigour, thus Vritra lay with scattered limbs dissevered.
There as he lies like a bank-bursting river, the waters taking courage flow above him. The Dragon lies beneath the feet of torrents which Vritra with his greatness had encompassed.
Then humbled was the strength of Vritra's mother: Indra hath cast his deadly bolt against her. The mother [Danu] was above, the son was under and like a cow beside her calf lay Danu.
Rolled in the midst of never-ceasing currents flowing without a rest for ever onward, the waters bear off Vritra's nameless body: the foe of Indra sank into enduring darkness.
Guarded by the Serpent stood the thralls of Dasas, the waters stayed like kine held by the robber. But he, when he had smitten Vritra, opened the cave wherein the floods had been imprisoned.
A horse's tail wast thou when he, O Indra, smote on thy bolt; thou, God without a second, thou hast won back the kine, hast won the Soma; thou hast let loose to flow the Seven Rivers.
Nothing availed him lightning, nothing thunder, hailstorm or mist which he had spread around him: When Indra and the Dragon strove in battle, Maghavan gained the victory forever.
Whom sawest thou to avenge the dragon, Indra, that fear possessed thy heart when thou hadst slain him, so that, like a hawk affrighted through the regions, thou crossedst nine-and-ninety flowing rivers?
Indra is King of all that moves and moves not, of creatures tame and horned, the Thunder-wielder. Over all living men he rules as Sovran, containing all as spokes within the rim.
he slew the Dragon: the Dragon is Vritra or clouds. In slaying the clouds, Indra brings rain. Cf. RV 1.57: "Thou [Indra], who hast thunder for thy weapon, with thy bolt hast shattered into pieces this broad massive cloud. Thou hast sent down the obstructed floods that they may flow."
Tvashtri: the gods' blacksmith.
he chose Soma: an intoxicating drink (often deified) with psychotropic effects. Indra, much like his Norse counterpart Thor, is a mighty imbiber of intoxicants, and soma itself often yields the Indo-Aryan equivalent of berserkr rage: e.g. "In the wild joy of soma I demolished Shambara's forts, nine-and-ninety together" (RV 4.26.3). Borne by an eagle, Indra brought Soma, for gods and men, from his/its heavenly (or perhaps mountainous) abode, another of his famous exploits, likely reflecting a common Indo-European myth -- Odin assumed the shape of an eagle after acquiring Kvasir's mead.
the shattered forts in falling: Perhaps a better word for "forts" would be "prison." The clouds have imprisoned the rain.
like kine/cattle held by the robber: the waters are likened to cattle. But the hymn is also alluding to another of Indra's exploits: his release of cattle, for the benefit of Aryan man, that had been penned up in the mountains by the Dasyus (RV 3.31 and elsewhere).
Nothing availed him: Vritra attempts to save himself from Indra through the use of magic; Indra also employs magic in the preceding verse, briefly becoming as thin as the hair of a horse's tail.
as spokes within the rim: The world as a wheel is one of the most common images in Hindu thought.
Rig Veda 1.32. Trans. Ralph T.H. Griffith, The Hymns of the Rig Veda (London, 1889), with minor modifications of the text and substantial expansions of the notes.