Ukraine
Before the U.S. lifted the ban on using their supplied weapons into Russian territory, Russia was able to load up troops, equipment and artillery right on the border near Kharkiv making their initial phase of the offensive successful. It was like they were taunting and abusing a pit bull on a chain that couldn't go beyond the length of the chain they were tethered to.
Then, the chain was detached. Now, Belgarod is the nearest Russian staging point and the tables have turned.