The NA has hidden behind US air cover and then with some minimal mechanized infantry and a few tanks (and like the Taliban, trucks and cars) overrun and held much of the country. Looks like they got Russian and Iranian equipment to do so.
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If you can trust what I've been reading in the Russian media and elsewhere, the NA rolled into Kabul on Russian tanks and Russia has given them a $70 million arms deal of tanks and armored personnel carriers (paid for by the US). Russian advisers and trainers were sent and there are reports that Russian military personnel were driving tanks and piloting helicopters.
The Northern Alliance is a Russian client army (as much as any force in Afghanistan can be said to be somebody else's client army). Russia has backed the Northern Alliance since the Taliban took power, and periodically unmarked bombers have been seen flying south over the Afghan border...
Chris Doss The Russia Journal