As the eighth day of Russia's invasion of Ukraine began Thursday morning, Russian forces appeared to have gained tactical control of their first city, the southern port city of Kherson, but Ukraine is still holding out in Mariupol, Kharkiv, and Chernihiv, despite heavy shelling. Deaths are mounting on both sides.

Big explosions were heard in Kyiv overnight, but according to the British Defence Ministry's Thursday morning update, the main body of the 40-mile-long Russian military convoy advancing on the capital remains nearly 20 miles from the city centre, "having been delayed by staunch Ukrainian resistance, mechanical breakdown, and congestion. The column has made little discernible progress in over three days."

Pentagon spokesman John Kirby gave a similar prognosis on Wednesday, saying the "stalled" column hasn't, "from our best estimates, made any appreciable progress in the last 24-36 hours," possibly because the Russians are "regrouping themselves and reassessing the progress that they have not made and how to make up the lost time," but probably also due to "logistics and sustainment challenges" and "resistance from the Ukrainians."

Trent Telenko, a retired Pentagon staff specialist and military history blogger, suggests another big reason may be Russia's tires, as he explained in a long, illustrated Twitter thread based on photos of deserted Russian Pantsir-S1 wheeled gun-missile systems and his own experience as a U.S. Army vehicle auditor. "When you leave military truck tires in one place for months on end," the sidewalls get brittle in the sun and fail like the tires on the Pantsir-SR, he wrote. "No one exercised that vehicle for one year."

Karl Muth, an economist, government adviser, and self-described "tire expert," jumped in, agreeing with Telenko but adding some details about the tires.

"There is a huge operational level implication in this," Telenko said. "If the Russian Army was too corrupt to exercise a Pantsir-S1, they were too corrupt to exercise the trucks and wheeled [armoured fighting vehicles] now in Ukraine," meaning "the Russians simply cannot risk them off-road during the Rasputitsa/mud season." That is a problem for the convoy in the north, he added. "The Crimea is a desert and the South Ukrainian coastal areas are dryer. So we are not seeing this there. But elsewhere the Russians have a huge problem for the next 4 to 6 weeks."