US and other Western troops (not NATO per se, though I think all were NATO members) were openly present in Ukraine training the Ukraine military up until shortly before the recent Russian escalation of their long-running invasion of Ukraine (they were pulled out to avoid getting caught up in direct combat with Russia in the event of an invasion.)

EDIT: if you need a source, here's an article on the US/UK trainers being withdrawn: https://www.forbes.com/sites/annakaplan/2022/02/12/us-uk-pul...