Short answers, pulled from the story.
Operation Uranus was a Soviet strategic encirclement operation launched on the 19th of November 1942 on the Eastern Front, designed to trap Axis forces around Stalingrad. It resulted in the encirclement of the German Sixth Army, the Third and Fourth Romanian armies, and portions of the Fourth Panzer Army, with between 250,000 and 300,000 Axis personnel caught in the pocket.
Operation Uranus ran from the 19th to the 23rd of November 1942. The northern assault began at 07:20 Moscow time on the 19th of November; the encirclement was formally completed near the village of Sovetsky on the 23rd of November.
The Red Army allocated an estimated 1,100,000 personnel, 804 tanks, 13,400 artillery pieces, and over 1,000 aircraft for Operation Uranus. The force comprised 11 armies along with independent tank brigades and corps.
Romanian forces defending the German flanks lacked adequate anti-tank weapons. Their Czech-built R-2 tanks carried only a 37 mm gun, ineffective against Soviet T-34 armor, and their 37 mm PaK anti-tank guns were largely obsolete. The Third Romanian Army was spread across a 140 km front, and the Fourth Romanian Army covered a line no less than 270 km long, leaving both armies over-extended and unable to concentrate against the Soviet breakthrough.
The Soviet 26th Tank Corps seized the bridge at Kalach-on-the-Don by using two captured German tanks and a reconnaissance vehicle to approach the crossing and fire on the guards. Soviet forces broke into the town of Kalach by mid-morning on the 22nd of November 1942, allowing the northern and southern Soviet pincers to link up.
Hitler ordered the Sixth Army to hold its position in what he designated "Fortress Stalingrad" and refused to authorize a breakout. The Luftwaffe attempted to resupply the trapped force, but the daily requirement was at least 680 tonnes and the airlift never delivered more than a fraction of that; during the first half of December the Sixth Army received less than 20% of its daily supply needs.